analyzing cumulative environmental effects of agricultural land drainage in southern ontario, canada

_ A g r i c u l t u r e r~cosystems c r E n v i r o n m e n t

E LSEVI E R Agriculture. Ecosystems and Environment 53 (1995) 279-292

Analyzing cumulative environmental effects of agricultural land drainage in southern Ontario, Canada

H a r r y S p a l i n g * Department of Geography, University of Guelph, Guelph. Ont. NIG 2WI, Canada

Accepted 12 October 1994

Abstract

The temporal and spatial attributes of agricultural land drainage suggest that drainage is a potential source of cumulative environmental change. This paper applies a cumulative effects model of drainage to the case of southern Ontario, Canada. The application is focused on t'~¢ cumulative effects of drainage on volume and timing of water flow, content of nitrate-nitrogen and atrazine residue in receiving channels, and area and distribution of wetlands. These are investigated using available information on drainage-environment interactions and geographic information system (GIS) technology. Available data show that the combined effect of two pathways of accumulation in drained areas (i.e. subsurface flow and runoff) i~tcreases the content of nitrate in water, but decreases the atrazine content, relative to the single pathway (i.e. runoff) in undrained areas. Drainage alters timing and volume of water flow at the field scale, but evidence of accumulation of these changes at the watershed scale is inconclusive for southern Ontario. This is attributed to counteractive processes and constraints of available data bases. Frag- mentary effects are evident in the spatial association between drainage density and change in area and contiguity of wetlands at the subregional and regional scales. Application of GIS demonstrated that 47% of the decline in total wetland area ( 13 !2 ha) in Peel Township during 1800-1990 is attributable to subsurface drainage. Average patch size decreased nearly 50%, and the number of large patches ( > 50 ha) declined by about 65%. In southern Ontario, counties with the greatest reduction in wetland area are consistently characterized by high drainage density. Collectively, the empirical application demonstrated the utility of the cumulative effects model, and showed that it is possible to conduct a cumulative effects analysis using available information sources.

Keywords: Canada: Cumulative effects: Environmental impact: Drainage: Agriculture

1. Introduction

This paper empirically applies a cumulative effects model of agricultural land drainage (Spaling and Smit, 1995) to the case of farm drainage in southern Ontario, Canada. The application tests the general hypothesis that drainage contributes to temporal and spatial accumulation of changes in environmental components and

*Corresponding author.

0167-8809195/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 1 6 7 - 8 8 0 9 ( 94 ) 0 0 5 6 7 - 2

processes. Available information on drainage-induced environmental change in southern Ontario is analyzed, interpreted and assessed relative to specific types of cumulative environmental effects hypothesized in the generic model.

Three indicators of environmental change are selected as a focus for the application: ( 1 ) hydrologic variables, particularly flow volume and response time; (2) water quality, s~ciflcally nitrate-nitrogen and atra. zine content; (3) the area and distribution of wetlands.

280 H. Spaling I Agriculture, Ecosystems and Environment 53 (1995) 279-292

Changes in water quantity and quality are selected because of their direct relationship to the fundamental purpose of drainage (i.e. removal of water from one location to another). Several studies have examined this relationship, providing an information base from which to obtain pertinent data. Wetland area and distribution are ~f interest because of the historic association between drainage tbr agriculture andthe decline of wetlands in southern Ontario. Few studies have examined this association, but those that have done so are generally characterized by long time peric.xls and broad spatial scales. Each drainage action is presumed to contribute to an incremental change in each environmental indicator. As drainage density increases, changes are hypothesized to accumulate ~dditively in water flow e.r.d water quality, and deerementally in wetland area.

Comprehensive data bases on the source, pathways and effects of drainage-environment interactions over broad temporal and spatial scales are not available for the study area. Thus, information is derived from a variety of sources (e.g. census data, aerial photographs, historical records), and from previous studies ofdrain- age--enviromrient interactions. Research findings from other locations are occasionally referred to, particularly when pertinent data from southern Ontario are lacking.

The analytical investigation focuses on two broad hypotheses derived from the conceptual model (Spal- ing and Smit, 1995). The first hypothesis, relating increases in drainage density to accumulation of environmental change, is assessed by reviewing evidence for the cumulative effect of drainage on volume and t.imi.ng of water flow, nitrate and atrazine loading in receiving waters, and area and distribution of wetlands. For example, drainage may gradually alter the timing of local or regional stream flow patterns, indicating time crowding, or time lags. Drainage may systematically gather eontamiaants which are geographically dispersed and deposit them at higher concentrations downstream, demonstrating spatial crowding. Spatial fragmentation or patchiness effects may be evident at the landscape scale in changes to area, pattern and contiguity of wetlands.

The second hypothesis stipulates that drainage- induced environmental changes accumulate from one scale to another. Evidence for scale aeeumt~lation is analyzed by first reviewing findings from field studies to establish changes at the micro scale, and then iden-

tifying evidence of accumulation of these changes at the meso or macro scales.

2. Cumulative effect of drainage on water flow

Concern regarding the cumulative effect of drainage on water flow in Ontario has been expressed by several researchers ( Found et al., 1976; Whiteley, 1979; Eddie, 1982; Bardecki, 1988). Underlying this concern is the assumption that changes in flow attributable to drainage at the field level accumulate over ti:oc and space, resulting in measurable changes to hydrologic processes at the watershed scale.

2. !. Flow characteristics o f f .eld drainage

Empirical evidence is available to describe the response of field drainage to precipitation and soil moisture contenL Classic drainage studies (e.g. Child,;, 1943; Goins, 1956; Schwab et al., 1961; Schwab and Thiel, 1963) sho~ that, in general, intense rainfall of short duration results in a hydrograph which peaks and declines quickly, whereas storms of lower intensity and extended duration exhibit a longer time to peak and a prolonged recession curve. Soil moisture content prior to onset of rainfall alters flow in one of two ways: during wet antecedent conditions peak discharge and volume are usually increased but, under dry antecedent conditions, these are normally reduced due to increased storage capacity in the soil. In addition, successive storms result in a complex response characterized by consecutive peak flows.

The~e generalized findings indicate that changes induced by field drainage are not simply summative. Drainage stimulates processes of accumulation which are counteractive. For example, removal of water from the soil profile increases flow volume, but also expands water storage capacity, which limits output. Processes of water removal and storage counteract each other to determine net change m output volume of a drained field.

Parallel processes offset the timing of drain flow: For example, drainage speeds up the rate of water movement from field to outlet but, because of greater siorage capacity, also increases retention time of intercepted flow in th.e soil. The rate of water movement and retention time cancel each oilier to some degree to regulate

H. Spaling /Agriculture, Ecosystems and Environment 53 (1995) 279-292 28 i

timing of drain flow. The net effect of these counteractive processes is less than the sum of the effects of each individual process.

Some data are available to compare surface runoff and subsurface drainage from drained and undrained fields. The relationship between these two variables is important because of their potential impact on total volume of flow, and on the proportion of flow through alternate pathways. Bengtson et al. (1988) analyzed total discharge from drained and undrained plots for 1981-1986 in Louisiana. Discharge volume from the drained plots (runoff plus subsurface flow) exceeded that of the undrained plots (runoffonly) for each year. Average annual output from the drained plots was 35% greater than the undrained plots. This finding supports the hypothesis that higher discharge from many drained fields may accumulate to increase total volume of flow from a watershed.

Drainage experiments have also investigated the effect of subsurface drainage on pathways of flow. Natho-Jina et ai. ( 1987 ) found that tile flow constituted a larger portion of total flow than runoff from drained fields in southern Quebec. The experiment by Bengtson et al. (1988) in Louisiana showed that drained plots reduced surface runoff by 34%. In Ohio, Schwab and Thiel (1963) observed that subsurface drainage decreased runoff by 40%. One explanation for this change in routing of flow is that a lower water table, and dry antecedent soil moisture, allow greater infiltration of rain water into the soil profile, which reduces surface runoff. Thus, drainage not only provides an alternate pathway of flow, but also alters th~ proportion of flow between pathways. Multiple pathways, and the varying effect of each, are compatible with the cumu- iative effects model.

This is fu,ther demonstrated in diitcring response times for the two pathways of flow. Natho-Jina et al. (1987) found that, for surface runoff time to peak and recession time are abrupt and short. In comparison, for subsurface drainage time to peak is delayed, and recession time is extended, Response time is greater for subsurface drainage because of larger flow volume, and a longer pathway for water to infiltrate the soil, perco- late into the drain, and flow to the outlet. Trig change in temporal response shows that drainage may result in ~Jme lags.

In summary, results of field drainage experiments confirm that subsurface drainage is generally charac-

terized by increases in total flow volume, modified pathways of flow, and lengthened flow response time. These findings are consistent with the hypothesis that drainage-inth~ced environmental changes accumulate over time and across space.

2.2. Changes to watershed f low

Studies which have investigated the effect of agricultural land drainage on watershed hydrology in southern Ontario are summarized in Table 1. These studies all consistently utilize data from watersheds with decadai records of precipitation and water flow, and historical evidence of increasing drainage density. They interpret drainage broadly to include boO, subsurface and arterial drainage.

An early study by McCubbin (1938) concluded that drainage did not contribute to flooding (i.e. maximum annual discharge) of the Thames and Grand Rivers (Table 1). Eddie (1982) used statistical procedures to analyze changes in flow for five watersheds, and associated these changes with precipitation patterns, but not drainage. Bardecki's (1988) study is notable because of its explicit focus on cumulative effects. Cumulative effect~ are interpreted as concurrent changes in drainage density and flow characteristics within a basin. This study also did not find any change attributable to drainage.

Serrano et al. (1985) conducted a detailed investigation into the effects of drainege on stream flow in the Middle Thames River for the period 1949-! 980 ( Table 1 ). This study also could not at~ributechanges in annual mean stream flow to drainage. Variations in stream flow closely followed fluctuations in annual mean precipitation. An analysis of the seasonal distribution of flow fu,,fller failed to manifest any association with drainage. However, temporal response of stream flow during storm events did change. The mean tt:me interval between total rainfall peak and total runoff peak for selected storms in the basin declined from 19 to 17 h during the 30-year period of study. Simil~arly, mean time from taart of runoff to runoff peak decreased from 18 to 14 h. These changes are consistent with the cumulative effect of temporal crowding.

Based on the four studies of 12 watersheds in Ontario, there are no conclusive data to demonstrate that changes to watershed hydrology ate attributable to drainage. However, results of these studies differ from

282 14. Spaling /Agriculture. Ecosystems and Environment 53 (1995) 279-292

Table I Summary of studies which have investigated the effect of agriculteral land drainage on watershed hydrology in southern Ontario

Watershed Data years Method of analysis Flow characteristic analyzed O b s e r v e d Reference change

Grand and 1 9 1 5 - 1 9 3 7 Historical Maximum annual discharge None McCubbin (1938) Thames

AtLsable 1949-1979 Moving average of ratio of Annual mean flow None Eddie (1982) Maitland 1948-1979 annual mean to long term Annual maximum daily flow None Saugeen 1916-1979 mean Annual minimum daily f l ow Inconsistent Sydenham (a) 1949-1979 Sydenham (b) 1949-1979

Middle 1949-1980 Double m~LSS curve, water Annual mean flow None Serrano et al. Thames balance, moving average, Seasonal distribution in flow None (1985)

variation in mean monthly Mean time to peak < 25% flow and runoff coefficient, lag time analysis

Historical Annual maximum daily flow None Annual maximum instant flow None Annual maximum daily and N,:ne instantaneous flow in spring Date-maximum daily flow None Date-minimum daily flow None

Ausable 1946-1987(? ) Concgtogo 1950-1987 Ganaraska 1950-1987 Nith 1947-1987 Nottawasaga 1949-198 Sangeen 1915-1987

Bardecki (1988)

research findings elsewhere. For example, Dybvig and Hart ( !977) conducted a simulation study of the effect of two existing and six proposed drainage projects (arterial drains) on flood peak discharge in eight sub- basins of the Moose Jaw River basin in Saskatchewan, Canada. Flood peak discharge increased in each sub- bas,.'n 1or a flood with a return probability of 25 years. The cumulative effect o f all drainage projects is an 184% increase in flood peak discharge o f the Moose Jaw River downstream of the projects. Although this study did not differentiate subsurface flow a~J runoff, it demonstrates me accumulation of changes in flow volume from one scale to another.

One explanation for the lack of empirical evidence in the Ontario studie~ is the role o f counteractive processes. For example, drainage may increase total discharge in one or more fields or sub-basins, only to be offset by decreased output from other areas because of variation in precipitation, soils, and drainage density throughout a watershed. Similarly, altered response time at one location is unlikely to be synchronized with that of all other locations. Processes of accumulation may counteract each other at one scale so that change is undetected at broader scales.

Another explanation relates to the methodological approach and data limitations in the Ontario studies. The history o f drainage precedes the instrumental record of precipitation and flow in southern Ontario. None o f the studies use a before and after approach because any trend analysis is restricted to time periods after the advent of drainage.

An additional explanation relates to the analytical focus on the rela ,onship between flow and precipitation. Any variation in flow which cannot be attributed to rainfall pattern is assumed to be drainage-induced, This approach masks any poterttial cumulative effect e f drainage on flow because d~inage is not explicitly quamified in the analyses. Two quantifiable, geographic variables which may dampen or amplify the cumulative effect of drainage on flow n_re .a~rea dra.ined and location of drained areas. A sufficient portion of the total area of a basin must be drained before signif- icant changes occur in stream flow (Irwin and White- ley, 1983). Trafford (1973) theorized that the relationshi~ between area drained and water discharge resembles a concave graph. This means that, as drainage density increases, peak discharge is reduced due to a decreasing ratio of surface runoff to subsurface d~'ain- age; but beyond an optimum point, discharge rates

H. Spaling ~Agriculture. Ecosyst¢.':ls and Environment 53 (1995) 279-292 283

increase again because subsurface drainage replaces runoff as the dominant source of flow.

Location of drained areas is an important consideration because those widely distributed throughout the watershed are likely to vary in their timing of water delivery to the receiving channel, consequently reduc- ing peak discharge (Whiteley, ! 979 ). Conversely, con- centrated areas of drainage occupying large portions of a watershed, especially upstream, are likely to show synchronized and quicker drain water delivery so that upstream and downstream flows merge, resulting in higher peak discharge.

To summarize, the lack of empirical evidence to verify cumulative effects of drainage on watershed flow in southern Ontario is explained by several factors. First, processes of accumulation are frequently counteractive, resulting in offsetting effects which are unde- ',coted at the watershed scale. Second, methodological approaches to date are restricted to trend analysis of changes to flow after drainage began. Before and after approaches, or comparative analyses of drained an6 undrained basins, have not been used. Third, the studies do not explicitly quantify drainage, or isolate its spatial effect.

3. Cumulative effect of drainage on water quality

Changes in water quality at individual sites are presumed to accumulate overtime and across space, resulting in measurable changes to water quality at the watershed scale. Since drained areas frequently receive inputs of inorganic fertilizer and herbicides, two indicators of water quality in these areas are nitrate-nitro. gen and atrazine residue. Data from available studies are reviewed for each indicator at the field and waler- shed scales.

3.1. Draiuage and nitrate-nitrogen

Drainage plot experiments, and studies of drained farm fields, in Ontario confirm that tile effluent contains varying concentrations of nitrate (Table 2). Concen- trations vary depending on soil and crop type, rate of fertilizer application, ann tillage system. Many exceed the acceptable maximum guideline for drinking water in Ontario (10 mg I-~), particularly those with high rates of fertilizer ~pplication. This empirical evidence

of nitrate in drain flow provides a basis for inferring spatial crowding and accumulation of nitrate at broader scales.

Results of studies which compare nitrate in discharge from drained and undrained fields generally indicate that nitr~te concentration and loading are increased by subsurface drainage. For example, in Ohio, Schwab et al. (1980) measured mean annual nitrate concentrations of 8.2 mg !- ' and 3.4 mg I - ' in discharge from drained and undrained plots, respectively. Nitrate loading from drained plots (18.7 kg ha-, per year) also exceeded that of undrained plots (12.1 kg ha- ' per year). This study, and others (e.g. Richard et ai., 1989), show that nitrate content and loading are higher in discharge water from subsurface drainage and runoff combined, than runoff alone. This illustrates an amplifying, or compounding effect.

The increase in nitrate attributable to drainage at the site level suggests that nitrate accumulates at the watershed ~ale. A comprehensive investigation of nitrogen levels in 11 asriculturai watersheds was carried out from 1975 to 1976 by the Pollution from Land Use Activities Reference Group (PLUARG) (Coote et al., 1982). Water quality data show a general positive relationship between tile drainage density and nitrate-nitrogen (NO.rN) concentration, and unit area loading, for most watersheds, particularly for densities above 15 (Table 3). Mean concentrations for these watersheds are less than the drinking water standard ( 10 mg NO3- N I- '), but greater than the suggested critical threshold of 0.3 mg I ' above wtfich eutrophication accelerates in nitrogen limited waters (Neilson et al., 1982). Stud- ies conducted elsewhere provide further evidence of drainage-induced nitrate loading at the watershed scale (e.g, Burwell et al., 1976; Lowrance et al., 1984). The empirical evidence is consistent with the hypothesis that changes in water quality which originate from field drainage accumulate at the watershed scale.

Subsurface drahtage affects nitrate content in water in several ways, each demonstrating a different type of cumulative elfect, First, drainage stimulates production of soluble nitrate-nitrogen because of increased aera- tion in the root zone. This represents a threshold or triggering effect. Second, nitrate content in water discharge from drained fields (subsurface drainage plus runoff) is generally higher than that of undrained areas (runoff only). The combined, or compounding, effect of two pathways of accumulation is greater than one

284 H. Spaling /Agriculture. Ecosystems and Environment 53 (1995) 279-292

Table 2 Concentrations of nitlate-nilrogca in the effluent for various studies in southern Onlario

Soil type Cropping pattern Fertilizer NO3-N Reference (kgha -1 per year) (mgl -t)

Bmokston clay Corn in rotation 448 14.0 BoRon et al. (1970) Continuous corn 448 8.9

Clay Corn in rotation 110 (85) ~ 17.5 Miller (1979) Clay 35 (50) 6A Sand 92 (104) 8.2 Sand Continuous corn 150 (150) 6.3 Clay 110 (130) 4.7 Sand 200 (150) 20.4 Brandon c l a y Hay--ceranl-corn 570 9.9

Corn-cereal- 101 8.7 alfalfa

Huron clay Various cash crop n/a 10.6 and live.stocR f~ms

Perth clay Whi~e beans 45 17.8 Corn 140 17.4

Sandy loam Plow tillage 160 1 I.I No tillage 160 10.1

Phillips et aL (1982)

Fleming (1990)

Fleming ( 1991 )

Kachanoski and Rudra (1992)

Walues in parenthe~s ~note recomrnende~ fertilizer rares, n/a, not available.

pathway (runoff). Third, drainage provides a route for nitrate-nitrogen to exit the soil profile and enter receiving waters. The transfer of nitrate from artificial to natural drainage systems illustrates cross-boundary movement. Finally, elevated nitrate loads and concentrations in basin sLreams generally are accounted for by relatively large areas in row crops, and high fertilizer

Table 3 Tile drainage density, nitrogen concentration and loading in I I agricultural watersheds in southern Oltt~-io. 1975-1977 (data from Come et al. (1982) and Neilson et al. (1980) )

Waten~hed Drainage no. density

(%)

NO3-N

Unit ~ a load Mean concentration (kg ha- I per year) (mgl -~ )

I0 0 5.2 2.13 2 2 6.4 1.05 7 5 2.4 0.57

14 13 4.8 0.88 I t 15 - 3.34 4 20 14.0 3.75 6 25 12.3 2.08 3 50 25.9 5.50 ! 80 15.3 5.62 5 98 22.8 4.33

13 99 i9.4 4.30

rates. Intensified land use and increased inputs exem- plify indirect cumulative effects attributable to drainage.

3.2. Drainage and atrazine

Field studies have confirmed that atrazine leaches down the soil profile and into drain water. Muir and Baker (1976) found a mean concentration of 1.2/xg I - ~ m diz~harge from four drainage plots in southern Quebec. Frank et al. ( 1991 ) reported mean concentrations of 7.8, 2.74, and i.9g/.tg 1- z in drain flow following first, second, and third atrazine applications, respectfully, on a 14 ha drained field in Ottawa. Con- finnatioh of atrazine residue in drain flow supports the hypothesis of accumulation at broader scales.

Frank et al. ( 199 ! ) also determined that tile drainage accounted for a small ~rcentage ( < 2%) of total atrazine removed after each application, but exceeded that of runoff. This is explained by increased infiltration in drained fields which redirects a substantial portion of surface runoff containing atraz~ne into the s~,il profile. Even though the percentage of atrazine residue removed in drain water is small, drainage effectively provides the only avenue by which atrazine accumulates in stream flow.

H. Spaling /Agriculture. Ecosystems and Er, vironment 53 (1995) 279-292 28;5

Table 4 Atmzine application and contamination in small alld large watershed.,, in southern Ontario (data froal Frank ( 1981 ), Frank and Sirons (1979). Frank et ai. (1982) and Frnak and Logan (1988) )

Watershed Area Period Total Samples Mean water Loading/loss to stream/fiver (kin z ) applied detect ing concentration

(MT) (%) (/zgl -~ ) (kg per basin (g ha-~ per per year) year)

Small watersheds a Canagagigue Creek 1.8 1975-1977 0.70 b 94 1.2 3.9 3.2 Means of l I 4.3 1975-1977 0.96 b 80 1.4 8.6 2. I watersheds

Large watersheds Grand River 679 ' 1975-1977 167.0 b na 0.4 903

1981-1985 155.1 c 91 1.3 2909. 4.2 Saugeen River 399 1975-1977 42.8 t' na 0.2 287

1981-1985 96.5 c 62 0.4 787 1.9 Thames Ri~?er 684 1981-1985 424.3 c 99 3.2 5561 8.1

a Eleven agricultural watersheds studied by the Pollution from 'land Use Activities Reference Group (.Frank et al.. 1982). na. not available; MT. lnetric tonnes. h Data for 1975. c Data for 1983.

The change in proportion of flow via subsurface and surface pathways implies that drained fields remove less a~azine than undrained fields. This is supported by Bengtson et al. (1990) in Louisiana where atrazine removed from drained plots (23.47 g ha - i ) was 55% less than undrained plots (51.64 g h a - I ) . An explanation is that greater infiltration on drained plots results in increased quantities of atrazine in the soil profile, where processes of chemical and biological breakdown contribute to dissipation before entry in drain water. Unlike nitrate, this shows a partitioning action of two pathways of flow (subsurface drainage and runoff), so that the combined effect is less than that of one pathway (runoff).

Evidence of atrazine at the watershed scale is shown in Table 4. Mean data for I I small agricultural watersheds in southern Ontario, including an example of Canagagigue Creek, verify that atrazine accumulates in basin streams. It is ~dso present at the mouths of three majoi rivers draining the intensely farmed areas of southern Ontario (Table 4). The highest concentration and loading of atrazinr, occurred in the Thames River basin. Mean concentrations and stream Ioadings have increased between the periods 1975-1977 and 1981- 1985 for the Grand and Saugeen Riv.~,'rs, This finding is consistent with the cumulative effect of time crowding because the time required for these river systems to

assimilate or recover from atrazine contamination appears to be decreasing.

These watershed studies indicate an accumulation of atrazine residue in small watersheds throughout soutl~- ern Ontario, and the region's main river basins. In this respect, the hypothesis of scale accumulation is supported. However, the relative effect of drainage on atrazine content at this scale is not fully known. At the field level, subsurface drainage has a dampening effect on atrazine residue in water relative to runoff from undrained fields. A similareffect can be expected at the watershed scale. This would indicate a partitioning effect in which net change resulting from two pathways of flow is less than that of one pathway. Empirical evidence to test this effect requires comparative studies of atrazine loading in watersheds with varying proportions of drained and undrained areas.

In summary, drainage-induced changes to waler quality are charaete~zed by multiple sources, alternate pathways, and various cumulative effects. Multiple sources of cumulative environmental change are evident from data which show that drainage contributes both atrazine and nitrate in water at field and wat,:rshed scales.

Drainage alters pathways of flow which affect processes of accumulation in different ways. Drainage amplifies accumulation of nitrate, but dampens it for

286 14. sparing I Agriculture, Ecosystems and Environment 53 (1995) 279-292

atrazine, demonstrating that the same pathways of flow can result in different effects on water quality. Although the compounding effect of two pathways of flow is dissimilar for each element, other cumulative effects are parallel. Time crowding is apparent because the interval between successive (i.e. seasonal, annual) applications of citrate fertilizer and atrazine herbicide is less than that required for basin streams to recover to undetectable levels. Spatial crowding is evident in the systematic gathering of nitrate and atrazine residue, each widely dispersed. Cross-boundary movement is manifest in the deiive.,y of both elements to natural water gystems. Drainage frequently shifts cropping patterns to monoculture corn which is associated with nitrogen fertilizer and atrazine use, indicating an indirect cumulative effect.

4. Wetlands

Agricultural land drainage is frequently associated with changes to wetlands in southern Ontario (e.g. Cecile et at., 1985; Snell, 1987; Bardecki, 1988). These include physical disturbance to vegetation and soils, lowerir,.g oftbe water table, and alteration of hydrologic processes. Changes to wetlands may accumulate over • ,/;me and space resulting in altered ecological functioning at the landscape scale. Disruptions may include decreased water storage and stream recharge, dimin- ished filtration capacity to maintain water quality, and reduced habitat and biotic diversity.

Several studies note that hie degree of disruption to regiol~al ecological functioning depends on spatial attributes of size and contiguity of wetlands within a landscape unit (e.g. watershed) (Bedford and Preston~ 1988; Johnston et al., 1988, 1990; Gosselink and Lee, 1989). Changes in size and contiguity are indicative of the cumdative effect of patchiness or fragmentation.

This section analyzes the effect of drainage on spatial changes in wetlands of southern Ontario, and assesses these changes relative to fragmentary effects. Drainage is interpreted more bro~/dly in this section to include arterial drainage systems because information sources usually do not differentiate the relative effect of arterial and subsurface drains on wetlands. Geographic information system (GIS) technology is used to empirically investigate the spatial association between drainage and wetlands for a selected township (Peel) and, more

generally, for counties of southern Ontario. The focus is on subregional and regional scales because spatial fragmentation is generally a landscape, rather than field, phenomenon. Also, more information is available at these scales.

4.1. Method to analyze spatial change in wetlands

Sneli's ( 1982, 1987) approach is used to construct a series of maps depicting wetland area and distributi..m at selected intervals. Spatial data on wetlands in Peel Township are obtained from Environment Canada (1985), and checked against soil capability maps of the Canada Land Inventory, and topographic maps. This information is digitized using mapping software ( A ~'VLAS*GIS ) to produce a base map delineating wetland area and distribution in the township circa 1800. Spatial changes in wetlands are digitized directly from aerial photographs for 1930,1967,1982 and 1990. This provides a data base from which to calculate changes in total area, number of wetlands, average patch size, and distribution of wetlands according to size for each time period. These measures are used to demonstrate the cumulative effect of fragmentation.

GIS is also employed to produce a map showing agricultural land drainage in Peel Township in 1991. Spatial data on the extent and type (systematic, ran- dom) of subsurface drainage, and location and length of municipal drains, are derived from existing maps (Ontario Ministry of Agriculture and Food, 1991 ), and checked against Peel Township records. This drainage map is superimposed on the wetlands map to determine change in wetland area attributable to drainage. Changes in areas which were formerly wetlands, but are currently drained, arc presumed to be related to drainage. This spatial association is determined only for the entire study period (1800-1991) because d.~ta on tbe spatial expansion of drainage in Peel Township for other intervals are not available.

At the reg;onal scale, data from Snell (1987) are incorporated into the same mapping software to gen- erate a map of sou',hem Ontario depicting changes in wetland area from 1800 to 1982 by county. This map is then superimposed on a map of southern Ontario showing area of subsurface drainage by county as of 1991. A 9 year difference in the terminal dates of these maps ( 1982 and 1991 ) is considered insignificant relative to a time span of nearly two centuries over which

H. Spaling ~Agriculture. Ecosystems and Environment 53 (1995) 279-292 287

Lake

Huron

-)

/ / C ~ • ~ f WeIlartd C . . . . . ge

~ i'/" Wetlancl Boundary in 1800 ~ / [ ] Wsllana Zoundat'." in 1990

Fig. !. Agricultural land drainage in Peel Township, 1991. Data from Ontario Ministry of Agriculture and Food ( 1991 ).

drainage e×panded and wetland area declined. The overlay of maps is used to identify any spatial association between drainage density and change in wetland area. This association provides a broad indication of cumulative fragmentation effects at the regional scale.

4.2. Spatial changes to wetlands in Peel To.reship

Spatial distribution of drainage in Peel Township for 1991 is shown in Fig. 1. More than one-half of the township is tile drained, most of it systematically. Arte- rial drains total 74 kin. Drainage density (55%) is moderately high relative to other drained areas in southern Ontario. Historical data on the spatial pattern of drainage expansion in Peel Township are not available, but general trends based on ,,he record of drainage

expenditure suggest that expansion ofsuosurface drainage was most intense daring 1971-1985, following several de,ades of high expenditure on construction of arterial drains. This i~ generally supported in a study of the evolution of drainage in nearby EIma Township (Irwin, 1986). This study found that construction of arterial drains predominated prior to the mid-1960s, &id that installation of ~ubsm"f.ace dr~ns be,-ame the major drainage activity after this time.

Change in wetland area and distribution in Peel Township for 1800-1990 is shown in Fig. 2. Total wetland area decreased by 1312 ha (60%) during this period. An overlay of drak, age and wetland maps shows that 621 ha (47%) of this decline is a~sociated with subsurface drainage. Most of the remaining loss is probably due to arteria I &alas because the ~eatest

288 H. Spaling /Agriculture. Ec¢:systems and Enviromnent 53 (1995) 279-292

Fig. 2. Change in wetland area and distribution in Peel Township, 1800-1990.

6O

5O

40

E :~ 20

10

0 ~800 1930 1967 1982 1990

Year

I I Number of Patches 8 Averago Patch Size

5o

40~

g

1o

Fig. 3. C~,ange in number and average size of wetland patches in Peel Township, 1800-1990.

H. Spaling /Agriculture, Ecosystems and Env#onment 53 (1995) 279-292 289

mtmtlt 5

0

y~"

Patch Size [ha I

Fig. 4 Distribution of wedand patches by size in Peel Township, 1800-1990.

Fig. 5. Area of subsurface drainage ( 1991 ) and change in wetland area (1800.-1982) in southern Ontario by county. Data from Ontario Ministry of Agriculture and Food (1992) and ~;nell (1987).

decline in wetland area occurred prior to the mid- 1960s, a period dominated by arterial drain construction ( I~ in, 1986).

Drainage contributes to spatial fragmentation of wetlands i~z PeeIT~i~hip. The number of wetland patches decreased slightly, but aveiage patch size declined by about 50% (Fig. 3). An explanation is that severing of wetlands by drainage maintains the number of patches as small wetlands are eliminated, and that the decline in area of large wetlands decreases average patch size,.

Further confirmation of fragment3ry effects is apparent in changes to the distribution of wetland patches by size (Fig. 4). The number of large patches ( > 5 0 ha) decreased by about ~,3%, whereas the number of mod- erate ( 16-50 ha) ard small ( < 15 ha) patches varied little. Again, this is explained by the severing effect of drainage.

This study of Peel Township demonstrates the fragmentary effect of drainage on wetlands at the subregional scale. Drainage reduces total wetland area and

290 H. Spaling /Agriculture, Ecosystems and Environmetu 53 f 1995) 279-292

average patch size, ae, d results in increased spatial fragmentation of wctlands. Accumulation of these changes suggests similar fragmentation at the regional scale.

4.3. Spatial changes to wetlands in southenl Ontario

Findings from Snell (1987) confirm changes in the area and distribution of wetlands in southern Ontario by county for ! 800-1982. Wetland area decreased from an estimated 23 800 km 2, or 25% of the total region, in 1800, to approximately 9330 km 2, or 10% of the total area, in 1982. Decline in wetland area is highest in the southwestern counties in both absolute and propor- tional amounts.

A spatial association between this wetland loss and drainage is demonstrated by superimposing change in original wetland area onto a choropleth map depicting percent of total farmland drained (drainage density) for each county (Fig. 5). Counties with the greatest decline in wetland area are ~onsistently characterized by high drainage density (e.g. southwestern Ontario). Counties with the largest remaining area of wetlands are spatially related to low drainage density (e.g. north- eastern Ontario). These associations have altered the spatial pattern of wetlands throughout the region. This is indicative of the fragmentary effect of drainage on wetland area at the regional scale.

This analysis provides some empirical evidence to demonstrate fragmentation in the area and distribution of wetlands at two scales. At the subregional scale, fragmentary effects of drainage are apparent in changes to total wetland area, average patch size, and dlstri't~a- tion of wetlands by size. Evidence of fragmentation at the regional scale is based on a strong spatial association between wetland loss and drainage density. This association has changed the pattern of wetlands in southern Ontario.

S. Conclusion

Available data on the effect of drainage on water flow, water quality, and wetland area broadly support the cumular~-e effects model. In general, the analysis demons~.r~tes that drainage-induced changes in environmental indicators a': one scale accumulate and manifest themselves at a broader scale. The evidence indicates that changes in water quality at the field level

accumulate at the watershed scale, and that drainage alters the area and pattern of wetlands at subregional and regional scales. However, there is not good evidence of changes in watershed flow attributable to field drainage in southern Ontario, despite findings elsewhere.

Accumulation of changes in timing and volume of flow, nitrate and atmzine content in water, and area and distribution of wetlands demonsuate that drainage results in various types of cumulative effects, l ime lags are apparent in extended recessien time of drain flow. Time crowding is evident in decreased response time of wate~bed flow following storm events. Elevated concentrations ot nitrate and atrazine in water indicate spatial crowding of elements originally dispersed across space. The spatial transfer of these elements exhibits cross-boundaxy movement. The amplifying effect of drainage on nitrate content demonstrates the compounding of two pathways, whereas the dampening effect on atrazine content indicates a partitioning effect. Altered cropping patterns on drained land demonstrate an indirect effect. Fragmentary effects are evident in changes to the number gad size of wetland patches, and the areal pattern of g etla~ds. Evidence of these cumulative effects contribute to an empirical knowledge of drainage-environment interactions.

The analysis shows that it is possible to identify and analyze cumulative effects, including temporal and spatial accumulation, using data which have been com- piled without explicit consideration of cumulative environmental change. This was done by reorganizing and interpreting available data with respect m the conceptual model of c,mulative effects. The utility of this approach was constrained by inadequate empirical information on the spatial effect of drainage on web lands. Application of GIS provided a useful tool to investigate the spatial association oetween these variables. The potential of this tool for the analysis of cumulative spatial changes is considerable.

Although this case study demonstrates that cumulative effects can be analyzed using available information sources, empirical evidence is scanty, and quantitative analyses of these effects is hindered by insufficient data. There is a need for long term monitoring of environmental change at various scales. Also, as the case of d~i~iage shows, there is a need to improve data collec- tion on the temporal and spatial at~butes of human activities.

14. Spaling ~Agriculture, Ecosystems and Environment 53 (1995) 279--292 291

The data requirements for a comprehensive empirical analysis of cumulative effects o f human-environment interactions are considerable. Measurement of cumulative sources, pathways and effects requires a temporal scan o f long duration, and geographic repre- sentation at various scales. New technologies such as remot~ ~unsii,g aiid GIS programs offer the potential to establish and analyze large data bases, but the nature of this information is primarily suitable for an analysis of cumulative effects characterized by spatial accumulation (i.e. change in wetland area, but not changes in timing or volume o f water flow, or nitrate and atrazine accumulation in streams and lakes). Other monitoring systems need to be established to set up data bases which can be used to describe and explain tem- porally variable processes of accumulation.

Acknowledgemen t s

This paper ha., ~'eceived financial support from the Social Sciences and Humanities Research Council o f Canada, the Ontario Ministry o f Agriculture, Food and Rural Affairs and the Eco-Research Program of the Tri- Council of Canada via the Agroecosystcm Health Pro- ject at the University of Guelph.

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analyzing cumulative environmental effects of agricultural land drainage in southern ontario, canada

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