Mathematics and Computers in Simulation 33 (1992) 427-432 Nonh-Holland

421

MINIMISING SOIL EROSION AND RUNOFF BY MAXIMISING CROPPING OPPORTUNITIES

C.Carroll’ M Littleboy M Halpin’ Land Management Research, QDPI, Emerald, Qld,

Land Management Research, QDPI, Brisbane, QLD.

1. INTRODUCTION

Rainfall is the major limitation to cropping on the central highlands of Central

Queensland. The regions annual rainfall is extremely variable (200 to 14Ot?mm), with the long-

term average 639mm.

The region is characterised by periods when there is insufficient rainfall to grow a crop

(1 in 10 years), and times when there is very high ra!nfall that causes severe soil erosion. In

this environment the challenge is to find and adopt a farm management practice that can

maximise both crop water use and minimise soil erosion, whilst maintaining profitability.

However, The development of appropriate technologies and testing of cropping systems

is complicated by season-to-season variability. Experiments to test different practices must be

run over many seasons to obtain reliable results. This is expensive and in many cases

impractical. A -way to overcome these problems is to use a model that can simulate the plant-

soil-water and farm management interactions.

One such model has been developed by Queensland Department of Primary Industries

and is known as PERFECT, (Productivity, Erosion and Runoff Functions to Evaluate

Conservation Techniques) [l]. It was developed to simulate the major effects of management

(e.g. crop rotation, fallow management, planting decisions) and climate on: soil water, runoff,

erosion, drainage, crop growth and yield. Daily climatic data is used as a basic input.

PERFECT can also be used to assess the risk and cost associated with different management

options.

The model has been based and validated on experimental results from num;rous studies

in Queensland including: variety and fertiliser trials, laboratory measurements, rainfall

simulator and catchment studies. One such catchment stttdy -was based at Capella on the

central highlands [2].

The model is used in this paper to: f compare a monoculture and an flexible opportunity cropping rotation,

* describe the effects of the two management optiors on the number of plantings, runoff,

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428 C. Carroll et al. I Mmitnising cropping opportunities

soil erosion, evaporation and crop water use, and

* evahtate the relative economic risk associated with opportunity cropping for different

soil water planting criteria.

Three surface management practices were also compared; zero, reduced and

conventional tillage. The simulations are considered for the three main crops grown in the

retion; sorghum, sunflower and wheat.

2. ASS IN THE SIMULATJ

2.1 Ccmtparison between Monoculture and Opportunity Cropping

In the simulations it was assumed: the soil type was a black cracking clay B Ug 5.12 [3],

with a 75 c_m soil depth, a 2% slope gradient and a slope length of 10 metres. All these

features are commonly found in the central highlands.

A plar+ng opportunity was possible provided there was: 25 mm rainfall for planting and

75 mm of acmmulated soil water available for crop use.

The planting periods used for the three main crops grown in the region were:

Wheat 15 April to 30 June

Sorghum 25 December to 31 January

Sunflower 1 February to 15 March

To account for delay following a planting rainfall, crops were planted six days after

these conditions were met.

Two types of simulations were run. Firstly, a cropping sequence that planted one

particular crop each year (monoculture cropping) and secondly, an opportunity cropping

sequence that planted either wheat, sorghum or sunflower given planting criteria were met.

Consequently, with opportunity cropping there is a potential to grow more than one crop per

year.

Simulations were also performed for three different tillage practices:

Zero tillage - herbicides are used to control weeds, with subsequent crops

planted into remaining crop stubble.

Reduced tillage- crop stubble is retained by using blade and chisel ploughs during a

fallow.

Conventional tillage- a disc plough incorporates crop stubble as a primary cultivation

and a chisel plough is used for remaining secondary cultivations.

In the simulations it is assumed that there are no nutrient or planting limitations.

C. Carroll et al. I Maximihg cropping opportunities 429

23

Five plant available soil water contents were compared to evaluate the relative risk

associated with opportunity cropping. The plant available soil water contents were: SO, 75, 100,

125 and 150 mm. The 150 mm soil water represents a full profile.

Current ccmmercial farm management costs and yield prices were used in the

comparison.

3. SI

3.1 Monoculture and opportunity cropping plantings

There were no differences in the number of possible plantings between the three tillage

practices. However, there were 30 more plantings with opportunity compared to monoculture

cropping, Table 1. In the paper the Reduced Tillage practlc( :s used to compare monoculture

and opportunity cropping. Reduced Tillage resembles the practlt:e ma.ny farmers are using on

the central highlands.

Predicted nilmber of crops planted for monoculture wheat, sunflower, sorghum and when using

opportunity cropping. Simulation was for 85 years of Central Highlands rainfall records.

onoculhre Opportunity Cl-oP (No. crops grown in $5 years) (Individual crops grown)

wheat 53 18 Sunflower 57 18 Sorghum 58 51

Total no. crops grown in 85 yrs = 87

In the monoculture simulation sorghum and sunflower, the two summer crops, had more

plantings than the winter crop, wheat. When oFportu&y cropping, sorghum was planted 51 out

of the 87 occasions. Sorghum had the benefit of the first planting option in summer.

Consequently, less sunflower crops were grown with opportunity compared to monoculture

cropping. Wheat was often doable rroF?ed after sorghum was planted in early December.

The extra number of crops grown with a flexible opportunity cropping rotation meant there

was an increase in crop water use.

32 Crop water use

Soil water content is the most important factor tkt dekrmines whcrhcr Gtiali till

430 C. Carroll et al, I Maximising cropping opportunities

produce runoff on black earths and many other soil types. The simulations showed that the

sequence am3 the number of crops grown, will have a major effect on the soil water content

and helIce the amount of runoff. Growing more than one crop a year will reduce runoff by

increasing the proportion of rainfall used as transpiration 141. Water use efficiency is

therefore, determined by crop seiection and cropping intensity.

However, even with opportunity cropping crop water use was only 29% of the water

balance, Table 2. .A relatively small proportion of the long-term rainfall. Interestingly7 runoff

was equiva!ent to 50% of the crop water use. This highlights that crop water use and soil

water storage is often exceeded in very high rainfall periods.

TABLE 2

Predicted crop water use, drainage, evaporation, runoff and soil loss for three crops grown as

monoculture compared to opportunity cropping. Simulations were for 85 years of Central

Highland rainfall records. (Reduce tillage farm practice).

Crop Use Drainage Evaporation Runafl Soil Loss Crop (mm .yf*! (mm yr-‘) (mm ~-9 (mm yfl) (t ha-l yr-‘)

Wheat 102 25 361 144 15 Sunflower 102 15 364 150 26 Sorghum 151 13 350 116 10 opportunity 189 5 346 91 6

3.3 Soil evaporation

Soil evaporation was still very large (55% of the water balance) even with opportunitj~

cropping and reflects the large amount of ineffectual rainfall that occurs in the region Over

60% of the regions daily rainfall occurs in amounts less than 10 mm. In the Central Highlands

evaporation rates average 4 mm day1 in winter ;dnd 8 mm day-l in summer. So, such small

falls of rain are soon evaporated.

McCalla and Army 151 have found that a combination of residue levels greater than

6000 kg ha-’ and relatively frequent rainfall were necessary for stubble cover to reduce

evaporation over a fallow. A black earth soil requires a relatively large amount of water to

replenish the soil water deficit from air dry to field capacity, thus smell increments of soil

water do not penetrate below the zone of evaporation and are not effectively stored.

3. Sui? erosion

Sorghum had the

vegative cover over the

least soil erosion of the three crops compared. This was due to more

summer rainstorm period and less runoff because of increase6 crop

C. Carroll et al. 1 Mmimising cropping opportunities 431

water use. Sunflower had the highest soil erosion rate 26 t ha-’ year-‘, Table 2. Carroll and

Halpin, [6] have also shown that monoculture sunflowers increase the risk of high soil erosion.

There are two main reasons; firstly the crop produces low stubble co-Jer and secondly, the

fallow and seedbed preparation for sunflower coincides with the regions summer rainstorm

period.

An added benefit with opportunity cropping is the reduction in soil erosion.

Opportunity cropping had similar soil erosion to zero tillage wheat which had the least erosion

from all the management practices simulated, Table 3.

LE3

Soil loss for monoculture wheat for three tillage practices. Simulation was for 85 years of

Central Highlands rainfall.

Tillage Soil Loss (t ha-l yf’)

Conventional 21 Reduced 15 Zero 2

3.5 Maximising profitability

Figure 1 shows the relative gross margin return for a range of soil water planting

conditions.

0.2 t

t OM- 50 75 IDO 125 150

Mhimum soil water a? pianting (mm)

Figure I. The re?ative gross margins for a range of plant available soil water planting

criteria.

132 C. Carroll et a!. I Wximisiry cropping opportunities

A crop plant, with 75 mm plant available water, had the best gross margin. Crops

planted on both low (50 mm) and high (150 mm) plant available soil water conditions, had the

lowest gross margins. There were more plantings with the 50 mm soil water planting criteria.

However, many of the plantings failed to produce a crop or had poor crop yields because of

insufficient rainfall. Conversely, with the more conservative planting criteria, many cropping

opportunities were missed and overall fallow costs increased.

4. CoNCLUSICBN

The simulations from the PERFECT model have shown that opportunity cropping

increased crop water use and reduced runoff and soil erosion. However, even with opportunity

cropping only a sma!l proportion of the ratiall (29%) was used by a crop, and soil evaporation

was also very large. This highlights farmers should aim to get the most from the very variable

rainfall they receive on the central highlands. However, when taking a cropping opportunity,

there should be sufficient plant available soil water to support crop grow; until there is follow

up rainfall.

There were no differences in the water balance between tillage prac.tices. However,

zero tillage may have practical advantages when opportunity cropping; o~fiy a herbicide

application is required before planting. Whereas, with conventional tillage up to 3 to 4

cultivations are required to prepare a seedbed before planting. This consideration will be

e+lored in future simulations.

References 111 Littleboy, M., Silburn, D.M., Freebairn, D.M., Woodruff, D.R., and Hammer, G.L.

Perfect. A computer simulation model of Productivity, Erosion, Runoff Functions to Evaluate Conservation Techniques. QDPI Bulletin, QB 89005, 1989. Sallaway, MM, Yule, D.F., Lawson, D., Carroll, C., Fossett, G., Burger, P., and Nickson, D. Runoff and soil loss study Capella, Central Queensland. QDPI Project Report Q088012, 1988, Northcote, K.H. A fact& key for the recognition of Australian soils. CSIRO. Reilin. S.A., 1979. Bernt, RD., and White, B.J. A simulation based evaluation of three cropping systems on crack& clay soils in a summer - rainfall environment. Agricultural Meteordog: 16:211 - 229, 1976. McCa?a, T.M., and Army, T-J.. Stubble mlllch. farming. Advances in rigronomy. 13:125 _ 196, 1961. Carrr?ll, C., and Halpin, M. Managing sutiowers in an erosion prone environment: Proc. Amt. Sunfirower ASSOC., 9th Workshop, Kooralbyn, Qld. 176 - 182.

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