Irrigation with Effluent Water: Effects of Rainfall Energy on Soil InfiltrationA. I. Mamedov, I. Shainberg, and G. J. Levy*

ABSTRACTSeal formation at soil surfaces is significantly affected by raindrop

kinetic energy (KE). We hypothesized that the deterioration in sealpermeability of soils irrigated with effluents, relative to that of soilsirrigated with fresh water (FW), is affected by raindrop KE. Theeffects of four droplet KE levels (3.6, 8.0,12.4, and 15.9 kj m~3) onthe infiltration parameters of four Israeli smectitic soils that had beenirrigated with FW or effluents, were studied with a drip-type rainsimulator. At the lowest KE (3.6 kj m~3), final infiltration rate (IR)values for the FW-irrigated samples were in the range of 9 to 14 nunh ' and were significantly higher than the corresponding values forthe effluent-irrigated samples, suggesting that seals were not fullydeveloped at this low KE and that the irrigation water type playeda major role in determining soil permeability. At high KE(15.9 kJ m '), the differences between the final IRs of FW-irrigatedand effluent-irrigated samples of a given soil were small (<1.1 mmh '), suggesting that at high KE, the effect of drop impact overshad-owed the effects of water quality on the final IR. Rate of seal formationwas faster in the effluent-irrigated samples than in the FW-irrigatedones, regardless of rain KE. The sensitivity of all four soils to the useof effluents was the greatest at a rain KE of 8 kj in \ At both lowerand higher rain KE levels, the effect of effluents on the'final IR,relative to that of FW, was less severe.

THE USE OF SECONDARY-TREATED WASTEWATER (efflu-ents) for irrigation of cultivated fields has recently

become a common practice, especially in regions suffer-ing from a shortage of FW. Effluents differ from theirfresh source water by their higher electrolyte concentra-tion and by the presence of dissolved organic matter andsuspended solids. In Israel, the total salt concentration ineffluents is 17 to 20 mmolc L"1, which is about twicethat of FW (8-9 mmolc L~'); there is also an increasein the sodium adsorption ratio, from 2.5 in FW to 5-8in the effluent (Feigin et al., 1991). Irrigation with waterof a moderate sodium adsorption ratio (=6) leads tosoils with exchangeable sodium percentages (ESPs) ofa similar value (USSLS, 1954). The hydraulic propertiesof soils having such an ESP are not likely to be affectedduring the irrigation season, but could deteriorate whenthese soils are leached with distilled water (DW), usedto simulate rain water.

The levels of dissolved organic matter and suspendedsolids in effluents depend on the quality of the rawsewage water and the degree of its treatment. Suspendedsolids present in effluents may accumulate and physi-cally block water-conducting pores, thereby leading toa sharp decrease in soil hydraulic conductivity (de Vries,1972; Rice 1974; Vinten et al., 1983). With respect to

Inst. of Soil, Water and Environmental Sci., Agricultural ResearchOrganization (ARO), The Volcani Center, P.O. Box 6, Bet Dagan50250, Israel. Contribution from the ARO, The Volcani Center, BetDagan, Israel. No. 638/1999 Ser. Received 17 Feb. 1999. "Correspond-ing author (vwguy@agri.gov.il).

Published in Soil Sci. Soc. Am. J. 64:732-737 (2000).

the possible effects of dissolved organic matter on thesoil, a number of studies have shown that its presencein effluents enhanced soil-clay dispersivity, increasedthe clay flocculation value (e.g., Durgin and Chancy,1984; Frenkel et al., 1992; Tarchitzky et al., 1993,1999),and was considered responsible for a decrease in thehydraulic conductivity of a sandy loam soil (Tarchitzkyet al., 1999).

The formation of a seal at soil surfaces exposed to thebeating action of raindrops is a common phenomenon inmany cultivated soils, worldwide. Surface seals are thin(<2 mm) and are characterized by greater density,higher shear strength, finer pores, and lower saturatedhydraulic conductivity, compared with those of the bulksoil (Mclntyre, 1958; Bradford et al., 1987). Seal forma-tion is caused by two mechanisms: (i) a physical break-down of soil aggregates caused by the mechanical impactof waterdrops; and (ii) a physico-chemical dispersionand movement of clay particles into a region 0.1 to0.5 mm deep, where they lodge and clog the conductingpores (Mclntyre, 1958; Agassi et al., 1981). The twomechanisms act simultaneously and the former en-hances the latter.

Surface sealing is significantly affected by the electro-lyte concentration of the soil solution at the soil surface(i.e., that of the applied water) and the ESP of the soil.Low electrolyte concentration in the soil solution andhigh ESP enhance clay swelling and dispersion, leadingto easier breakdown of the surface aggregates and tothe formation of a less permeable seal (Agassi et al.,1981; Kazman et al., 1983). Even at low ESP levels (<6),a small increase in ESP was reported to result in a sharpdecrease in the infiltration rate (IR) of the seal (Kazmanet al., 1983). Thus, it is expected that soils irrigated witheffluents, and subsequently having ESP levels of =6,will exhibit a higher susceptibility to seal formation thansoils irrigated with FW.

The physical breakdown of surface aggregates (i.e.,the first mechanism) is determined also, to a large ex-tent, by the KE of the waterdrops (Moldenhauer andKemper, 1969). In soils with stable aggregates, high-KEwaterdrops were needed to form a seal (Agassi et al.,1985). In unstable soils, a seal may be formed underlow-KE waterdrops by the process of fast wetting andaggregate slacking (Le Bissonnais, 1990). Studying theeffects of drop impact energy in the range of 3 to 24 kJm~3 on two Israeli non-sodic loamy soils, Betzalel et al.(1995) found that with an increase in raindrop KE, theIR for any given rain depth decreased. Shainberg andSinger (1988) observed that sodic soils were more sus-ceptible to sealing by low-KE raindrops than wereCa-soils.

It is hypothesized that the susceptibility to seal forma-

Abbreviations: DW, distilled water; ESP, exchangeable sodium per-centage; FW, fresh water; IR, infiltration rate; KE, kinetic energy.

732

MAMEDOV ET AL.: EFFECTS OF RAINFALL ENERGY ON SOIL INFILTRATION 733

tion of soils that had been irrigated for long periodswith effluents would depend on raindrop KE. Whenexposed to high-KE rain, effluent-irrigated and FW-irrigated soils should show similar sensitivities to sealingand a low IR because the KE of the raindrops is highenough to disintegrate the aggregates in both types ofsoils. Conversely, under conditions of low-KE rain,when the physico-chemical clay dispersion (the secondmechanism of seal formation) is the more importantmechanism in determining soil sensitivity to sealing, ef-fluent-irrigated soils should show higher susceptibilityto seal formation and lower IR than soils irrigated withFW. The objective of our study was to examine theabove hypothesis by studying the effects of the KE ofwater droplets on the infiltration parameters of foursmectitic soils that had been irrigated for >15 yr withFW or effluents.

MATERIALS AND METHODSSoils

Four calcareous smectitic soils, representing the main arablesoils in Israel, were chosen for this study: a loamy loess (CalcicHaploxeralf) from Be'er Sheva Valley; a dark brown sandyclay grumusol (Chromic Haploxerert) from Hafetz Haim, thePleshet Plains (grumusol HH); and two dark brown heavyclay grumusols from Yagur, the Zevulun Valley (grumusol Y)and Eilon, the Western Galilee (grumusol E). Samples of thecultivated layer (0-250 mm) of each soil type were taken fromadjacent fields irrigated for >15 yr, one with FW and theother with effluents. Selected physical and chemical propertiesof the soils, determined by standard analytical methods (Klute,1986; Page et al., 1986), are presented in Table 1. The proper-ties of the effluent used for irrigating each soil type are pre-sented in Table 2. For the ions, the values given in Table 2are the actual values in the effluents used for irrigation becauseelectrolyte concentration and composition did not change sig-nificantly in the past years. The organic load (biological oxygendemand [BOD], chemical oxygen demand [COD], and totalsuspended solids) in the effluents fluctuated over the years,thus the values given represent data obtained in 1998. Regard-ing the FW, the samples were taken from fields that wereirrigated from the same source of FW, namely the nationalwater carrier of Israel.

Infiltration StudiesInfiltration rate was studied with a drip-type rainfall simula-

tor. The simulator consisted of a 750- by 600- by 80-mm closedchamber in which rainfall of a known constant drop size was

Table 1. Some physical and chemical properties of the soils used.

Table 2. Properties of the irrigation water at the various sites.Effluent

Index

PHEC, dS m 'Cl, mg L 'HCO3, ininol, L-'Na, iiinuil, L^1

Ca + Mg, ininol, L 'SARf, (mmol. L^1)05

BOD!, mg L 'COD§, mg L~'Suspended solids, mg I. '

Loess

7.51.7

200.0nd9.57.75.1

65.1296.5116.7

Hafetz Haim

7.91.9

334.16.78.26.05.3

21.691.640.5

Yagur

7.42.3

453.37.5

11.97.06.4

143.0ndflnd

Eylon

nd1

1.8205.8

nd7.5nd3.2

13.66734.5

rresiiaverage

7.40.95

2205.04.65.02.9

t SAR, sodium adsorption ratio.:j: BOD, biological oxygen demand.§ COD, chemical oxygen demand.II nd, not determined.

generated through a set of hypodermic needles (=1000) ar-ranged at a spacing of 20 by 20 mm and pointed downward.The average droplet diam. was 2.97 ± 5 X 10~2 mm. The KEof raindrops was varied by changing the height of fall of thedroplets. Heights of 0.4,1.0,1.6, and 2.2 m were used to obtaindrops with impact velocities of 2.5, 4.02, 4.98, and 5.64 m s"1,respectively (Epema and Riezebos, 1983). The correspondingenergies of the drops were 3.4 (low), 8.0 (intermediate),12.4 (medium), and 15.9 (high) kj m'3. The latter energy levelis commonly obtained in rainstorms typical to Mediterraneanclimates (Betzalel et al., 1995). Rain intensity was maintainedat 36 mm h"1 by a peristaltic pump.

Air-dried aggregates, crushed to pass through a 4.0-mmsieve, were packed in 200- by 400-mm trays, 40 mm deep,over a 10-mm thick layer of coarse sand. The trays were satu-rated with tap water (electrical conductivity = 0.9 dS m"1 andsodium adsorption ratio [SAR] = 2.5) for 1 h at a matricpotential of —0.2 kPa and were then placed in the rainfallsimulator at a slope of 15% and exposed to 60-120 mm ofDW (electrical conductivity = 0.04 dS m"1) rain. During eachstorm, water infiltrating through the soil was collected for2-min periods, separated by 2-min intervals, in graduated cyl-inders placed underneath a special outlet at the bottom of thetray, and the water volume was recorded as a function of time.Infiltration water was collected until the volume of water inthe cylinders in three consecutive samples differed by no morethan 5%, indicating that the infiltration rate was approachinga steady-state value. Three replicates were used concurrentlyfor each treatment.

Data AnalysisInfiltration data obtained from the rainfall simulator were

analyzed with the nonlinear equation proposed by Morin and

Soil ClassificationSource of

irrigation water

Particle-size distribution

sand silt clay CaCOj CECf ESP! OM§

Loess

Grumusol(Hafetz Haim)Grumusol(Yagur)Grumusol(Eylon)

CalcicHaploxeralfChromicHaploxerertChromicHaploxerertChromicHaploxerert

fresheffluentfresheffluentfresheffluentfresheffluent

413 ± 17.7fl450 ± 21.2438 ± 17.5458 ± 19.6150 ± 6.4220 ± 7.8119 ± 5.5119 ± 6.3

•*£362 ± 17.7340 ± 19.5156 ± 5.3156 ± 3.8360 ± 16.1310 ± 18.2391 ± 14.9351 ± 17.6

225 ± 14,8210 ± 10.6406 ± 22.9386 ± 17.7490 ± 19.1470 ± 21.6490 ± 20.4530 ± 17.3

%18.0 ± 0.7418.4 ± 0.6110.7 ± 0.528.5 ± 0.43

16.4 ± 0.4514.3 ± 0.885.2 ± 0.277.3 ± 0.81

c inol, kg '17.7 ± 0.3917.5 ± 0.4334.2 ± 0.2933.4 ± 0.3756.3 ± 0.6551.4 ± 0.7371.2 ± 0.7666.2 ± 0.69

o/

2.1 ± 0.136.5 ± 0.212.3 ± 0.195.5 ± 0.152.2 ± 0.207.5 ± 0.551.5 ± 0.142.4 ± 0.19

2.1 ± 0.282.1 ± 0.213.4 ± 0.202.1 ± 0.183.4 ± 0.203.8 ± 0.416.0 ± 0.335.4 ± 0.42

t CEC, cation-exchange capacity.£ ESP, exchangeable sodium percentage.§ OM. organic matter.fl ± one standard deviation.

734 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

Benyamini (1977):

/, = (/i - /f) + I, [1]where 7, is the instantaneous infiltration rate (mm h~'); I, isthe initial infiltration rate (mm h"1); If is the final infiltrationrate (mm h"1); y is the soil coefficient related to surface aggre-gate stability (mm"1); t is the time (h) from the beginning ofthe storm; and p is the rain intensity (mm h~').

A nonlinear regression program used the measured /„ /,,and P values to calculate the other two parameters of theequation (7; and y) that gave the best coefficient of determina-tion (R2 > 0.9) between paired calculated and measured 7,values.

RESULTS AND DISCUSSIONIrrigation of the soils with effluents increased their

ESP from 1.5 to 2.3 (in FW) to 2.4 to 7.5 (Table 1).The sodium adsorption ratio of the effluents in westernGalilee (grumusol E) is exceptionally low (Table 2),therefore the ESP of this soil remained low even aftera long period of irrigation with effluents. In addition,except for the grumusol HH, irrigating for more than15 yr with effluents had no significant effect on theorganic-matter content of the soils (Table 1). Very inten-sive cultivation under a Mediterranean climate gener-ally prevented the accumulation of organic matter infields irrigated with effluents.

Seal formation is commonly characterized by changesin the IR with cumulative rain. The effect of raindrop

40

35-

30-icE 25-

£ 20-

| 15-

5-

0

Loess 35-

30-

25-

20-

15-

10-

5-

00 10 20 30 40 50 60 70 80 90 0

40-,——,——i—,——i——.—i—.——,——.—i—.——,—,—i——,—,——,——i 40

35-

30-icE 25-

S 20-I

1 10:5-

Grumusol (Yagur)

Water Rain energyfresh effluent kJ m3

-•- -D- 3.6-•— —o- 8.0-A- -A- 12.4-»- -0- 15.9

10 20 30 40 50 60 70 80 90

KE on the IR of the four smectitic soils, which had beenirrigated with effluent or FW, is presented in Fig. 1. Thedata indicate that both the KE of the waterdrops andthe quality of the water used for irrigation had strongeffects on the IR. In general, the IR of the soils de-creased with an increase in rain KE; and the IR valueswere lower for the soils irrigated with effluents than forthe soils irrigated with FW. However, the magnitude ofthese effects depended on soil properties.

Infiltration curves are not suitable for quantitativecomparison between treatments. Therefore, two param-eters were used to represent the infiltration curves: (i)the measured near-steady-state IR at the end of thestorm (final IR), and (ii) the soil stability coefficient (y)from Eq. [1], which represents the rate at which the IRdecreases and the seal is formed.

The mean final IR values of the various treatmentsfor the four soils are presented in Fig. 2. Results of amultifactor analysis of variance showed that each mainvariable (i.e., soil type, irrigation water quality, and rain-drop KE) significantly affected the final IR. Moreover,a significant interaction (P = 0.05) was observed (Table3) among the three variables (soil type X irrigationwater quality X raindrop KE) in their effect on the finalIR. Thus, a contrast test (SAS, 1995) was performed todetermine the differences among the final values ofindividual treatments.

For both FW-irrigated and effluent-irrigated samples,the final IR decreased significantly with an increase in

40

Grumusol (Hafetz Haim)

10 20 30 40 50 60 70 80 90

35-

30

25

20

15

10

5

Grumusol (Eylon)

10 20 30 40 50 60 70 80 90

Cumulative rain mm Cumulative rain mmFig. 1. Infiltration rate curves for the fresh water-treated and effluent-treated samples of the four soils subjected to the various rain kinetic

energy levels. Data points represent measured values, means of three replicates. Bars indicate ± one standard deviation.

MAMEDOV ET AL.: EFFECTS OF RAINFALL ENERGY ON SOIL INFILTRATION 735

rain KE in all four soils. This finding was expected, sinceincreased KE causes more aggregates to be disruptedand a denser, less permeable seal to be formed.

In soils irrigated with FW, the decrease in the finalIR with an increase in rain KE was more pronouncedin the loess than in the grumusols (Fig. 2). A similarbut less pronounced trend was noted in the effluent-irrigated samples. The sharp decrease in the final IRvalues of the loess with the increase in KE was mainlydue to the high final IR under low-KE rain (Fig. 2). Atlow KE, the impact of the waterdrops was apparentlynot sufficient to form a seal, thus the final IR was thendetermined by the infiltration rate of the soil profileand not by that of the soil surface. Under rain with lowKE, the IR of the loess, with 22% clay, was higher thanthat of the grumusols, with >40% clay. In our rainfallsimulation studies, the soils were exposed to fast wet-ting, leading to substantial aggregate slacking (Le Bis-sonnais, 1990; Levy et al., 1997). In grumusols with sta-ble aggregates, the hydraulic conductivity is oftensimilar to or higher than that of the loess (Levy et al.,1999), but upon fast wetting and substantial aggregateslacking their IR decreased to values lower than thatof the loess. Conversely, when high-energy rain(15.9 kJ m~3) was used and a developed seal was formed,the final IR of the loess was similar to that of the gru-musols.

14

&cflco

12-

10-

8-

4-

2-

0

-•- LoessGrumusQls

-•- Hatetz Haim-A-Yagur-*-Eylon

(a) Soils irrigated with fresh water

0 10 15 20

14

12-

10-

8-

6-

4-

2-

(b) Soils irrigated with effluent

0 10 15 20

Rain energy (kJ m )Fig. 2. Mean final infiltration rate as a function of rain kinetic energy

for (a) samples irrigated with fresh water, and (b) samples irrigatedwith effluents. Points labeled by the same letter do not differsignificantly at the 0.05 level.

For all four soils, when the lowest KE (3.6 kJ m 3)was used, final IR values for FW-irrigated samples weresubstantially higher than those for effluent-irrigatedones. This finding was ascribed to the differences be-tween the ESP of the FW-irrigated and effluent-irri-gated samples in the loess and the grumusols (HH andY) (Table 1). In the grumusol (E), the small differencein ESP between the FW-irrigated and effluent-irrigatedsamples (1.5 and 2.4, respectively) caused only a small(<1.2 mm h"1), yet significant difference in final IRbetween the two types of samples.

For the highest KE (15.9 kJ nT3), final IR values forthe four soils were in the range of 4 to 5 mm h~' forthe FW-irrigated samples and 3.5 to 4.5 mm h"1 for theeffluent-irrigated samples. This narrow range within agiven sample type indicated that when high-KE rain wasapplied, a developed seal was formed and the hydraulicproperties of the seals of the four soils were similar,leading to small differences in the final IR values amongthe soils. Furthermore, for all soils but the grumusol(HH), final IR values in the FW-irrigated samples didnot differ significantly from those in the effluent-irri-gated samples. These observations suggested that whenhigh-energy rain was used, the impact energy of theraindrops (i.e., the physical mechanism) masked the dif-ferences among soils irrigated with water of the samequality. Furthermore, in intensively cultivated soils,small differences in ESP (which contributes to the phys-ico-chemical mechanism) were not sufficient to affectseal properties when high-KE rain was applied, there-fore the final IRs of the soils were similar. Cultivationweakens the structural stability of soils, thus, mechanicalbreakdown of aggregates by the impact of high-KE rain-drops played a dominant role in seal formation.

The values of y (soil stability coefficient, Eq. [1]) aregiven in Table 4. In general, for a given soil and typeof irrigation water, the y coefficient values increasedwith increasing rain KE, indicating that the higher theKE of the rain, the faster the formation of the seal andthe decrease in IR. The y values for a given soil andrain KE were lower for the FW-irrigated samples thanfor the effluent-irrigated ones. This observation impliesthat the rates of surface aggregates breakdown and sealformation were higher in the effluent-irrigated samplesthan in the FW-irrigated ones. Shainberg et al. (1992)proposed that of the two mechanisms contributing toseal formation—aggregate breakdown and clay disper-

Table 3. Significance of effects of soil, irrigation water quality,and rain kinetic energy (KE) on final infiltration rate (IR).

Variable Source of variationSum of

DF squares F ratio Significance

Final IR SoilWater qualitySoil X water qualityRain KESoil x rain KFWater quality X rain KESoil X water quality

X rain KEErrorCorrected total

313393

96495

41.2760.769.95

380.8338.7014.06

3.208.51

557.28

103.45456.9424.95

954.6432.3435.24

2.67

##**#********#####

*

*, *** Significant at 0.05, 0.001.

736 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

Table 4. Mean soil stability constant (7, mm"1) at various rain kinetic energies (KE).

SoilWaterquality

Rain KE

3.6 8.0 12.4 15.9

Loess

Grumusol Hafetz Haim

Yagur

Eylon

fresheffluentfresheffluentfresheffluentfresheffluent

0.028 ± 0.002f0.033 ± 0.0040.067 ± 0.0040.073 ± 0.0050.060 ± 0.0050.073 ± 0.0030.031 ± 0.0010.042 ± 0.005

—————————————————————————————— KJ 11

0.065 ± 0.0060.086 ± 0.0030.099 ± 0.0050.174 ± 0.0090.100 ± 0.0180.135 ± 0.0210.072 ± 0.0040.109 ± 0.005

11 ———————————————————

0.115 ± 0.0040.138 ± 0.0040.158 ± 0.0180.299 ± 0.0380.172 ± 0.0170.216 ± 0.0280.115 ± 0.0100.116 ± 0.004

0.152 ± 0.0060.183 ± 0.0110.191 ± 0.0110.233 ± 0.0360.223 ± 0.0220.228 ± 0.0180.148 ± 0.0120.150 ± 0.015

t ± one standard deviation.

sion—the former is a rapid process and is completedbefore the full development of the seal; clay dispersionwas suggested to be the rate-determining mechanism ofsealing. The y values in the effluent-irrigated sampleswere higher than in the FW-irrigated ones, which sug-gested (as expected) that clay dispersion in the effluent-irrigated samples was enhanced by the higher ESP anddetermined the rate of seal formation.

To evaluate the relative sensitivities of the soils toirrigation with effluents, we calculated for each soil theratio of the mean final IR of the effluent-irrigated sam-ple to the mean final IR of its respective FW-irrigatedone for every rain KE (Fig. 3). The data presented inFig. 3 indicate that the loess and grumusol (HH) wereaffected to a greater extent than the grumusols (Y) and(E) by the use of effluents. The higher susceptibility ofthe former two to irrigation with effluents was attributedto their lower clay content (Table 1). Kemper and Koch(1966) suggested that clay acts as a cementing agent,stabilizing soil aggregates. The higher the clay content,the more stable the aggregates and therefore the higherthe resistance of the soil to seal formation. Similar find-ings were made by Ben-Hur et al. (1985), who foundthat soils with 20 to 30% clay were the most susceptibleto seal formation; those with clay content >40% hadstable aggregates and showed less sensitivity to sealformation (Ben-Hur et al., 1985).

-•— LoessGrumusol

-•- Hafetz Haim-A- Yagur

Eylon0.5

Rain energy (kJ m"3)Fig. 3. The ratio of mean final infiltration rate (IR) of effluent-treated

samples (FIRe) to mean final IR of fresh water-irrigated samples(FIRf) as a function of rain energy for the four soils.

All four soils showed the highest sensitivity to theuse of effluents for irrigation at rain KE of 8 kJ m~3,with less sensitivity at both the lower and higher rainKE levels studied. The high sensitivity of the effluent-irrigated soils at a KE of 8 kJ m~3 was explained asfollows. At KE lower than 8 kJ m~3, the effect of rain-drop impact on seal formation was small, and soil perme-ability was determined primarily by water flow throughthe soil profile (i.e., soil hydraulic conductivity). Differ-ences in ESP between =2 (FW-treated samples) and~6 (effluent-treated samples) do not have a substantialeffect on the hydraulic conductivity of calcareous soils(Shainberg and Letey, 1984). When such soils areleached with DW, CaCO3 dissolves at a rate high enoughto prevent clay dispersion and the decrease in the hy-draulic conductivity of soils with low to moderate ESPis limited (Shainberg and Letey, 1984). Therefore, onlysmall differences were observed between the final IRvalues of the effluent-irrigated samples and those of theFW-treated samples, and, therefore, the ratio betweenthem was high (Fig. 3). At KE levels >8 kJ m"3, therelatively high KE of the raindrops predominated indetermining the permeability of the seal formed. Theeffects of ESP on the permeability of the seal wereovershadowed by the rain KE. Consequently, differ-ences between final IR values of the effluent-treatedand FW-treated samples were relatively small, and theratio between the two was high (Fig. 3). But at KE of8 kJ m"3, the combined effects of the physical mecha-nism (rain KE) and the physico-chemical mechanism(differences in ESP) allowed a wide separation betweenthe final IR values of soils having different ESP levels.

SUMMARY AND CONCLUSIONSWe compared the susceptibility to seal formation of

effluent-irrigated soil samples with that of FW-irrigatedsoil samples for four calcareous soils exposed to rain offour different KE levels. For both the FW-irrigated andthe effluent-irrigated samples, final IR values decreasedwith an increase in rain KE, and the rate of seal forma-tion (i.e., the rate at which soil permeability declined)increased. At the lowest KE studied (3.6 kJ m~3), finalIR values were significantly lower in the effluent-irri-gated samples than in the FW-irrigated ones. At thehighest KE (15.9 kJ m"3), differences in final IR betweenFW-irrigated and effluent-irrigated samples of a givensoil were small and mostly insignificant. It was con-

MAMEDOV ET AL.: EFFECTS OF RAINFALL ENERGY ON SOIL INFILTRATION 737

eluded that when high-KE rain was used, raindropimpact energy was the predominant mechanism thatcontrolled seal formation and permeability; hence bothFW-irrigated and effluent-irrigated samples showedsimilar susceptibility to sealing. Differences in the soilsamples because of irrigation water quality (i.e., higherESP in the effluent-irrigated samples) contributed todetermining the IR of the seal, only when low-KE rainwas used; under these conditions, effluent-irrigated sam-ples emerged as more susceptible than FW-irrigatedsamples, to seal formation. More specifically, at a KEof 8 kJ irT3, samples irrigated with effluent were foundto be the most sensitive to seal formation comparedwith FW-irrigated samples. At the lowest KE (3.6 kJm~3), water flow through the soil was determined mainlyby the permeability of the soil profile and not by theIR of the surface layer. In calcareous soils, the formeris less sensitive than the IR to differences in ESP inthe range studied. At higher rain KE, rain propertiesdictated the soil susceptibility to sealing. Hence, forMediterranean type rainstorms, it is expected that sealformation of similar permeability will be formed in bothFW-irrigated and effluent-irrigated samples.

ACKNOWLEDGMENTSA.I. Mamedov is grateful to MASHAV, Israel Ministry of

Foreign Affairs, and the Agricultural Research Organization,Bet Dagan, Israel, for providing him with the funds that en-abled him to contribute to this work. This study was supportedby grant no. 302-0240-98 from the Chief Scientist, Ministry ofAgriculture and Rural Development, Israel. The support ofthe Chief Scientist is gratefully acknowledged.