HYDRAULIC PROPERTIES AND EROSION OF SOILS IRRIGATED WITH EFFLUENT WATER

HYDRAULIC PROPERTIES AND EROSION OF SOILS IRRIGATED WITH EFFLUENT WATER: EFFECT OF SOIL TEXTURE AND WETTING RATE

Mamedov Amirakh, Mikhailov Fariz

Institute of Soil, Water and Environmental Sciences, ARO, The Volcani Center, Bet Dagan, 50250, Israel.

ABSTRACT

In arid and semiarid zones the use of treated effluent water (EW) survey as a source of irrigation water. However long-term use of EW in cultivated lands may involve problems of soil salinization, deterioration of soils permeability by sodicity and organic colloids. Three experiment were conducted in order to evaluate the effect of long term irrigation with EW on the hydraulic properties (hydraulic conductivity - HC, infiltration rate -IR) and erosion of soils: (i) effect of water quality and rain energy; (ii) effect of soil texture (clay content 10-65%) and sodicity level; (iii) effect of gypsum (5 t/ha) and wetting rate.

Results showed that competency of irrigation with EW in changing hydraulic properties and seal formation strongly depends on soil texture, irrigation water quality and wetting rate. Irrigation with treated EW decreased HC and IR and increased soil susceptibility to dispersion and sealing processes. Soil irrigated with EW found to be more sensitive to rain energy, wetting rate and gypsum application than soils irrigated with fresh water (FW). The clay content and wetting rate predominated in determining aggregate stability and subsequent susceptibility of soil to sealing. In clay soil slow prewetting rate prevent aggregate slaking and sodicity related degradation for moderate level of ESP. On soils with clay content < 20-40%, the effect of gypsum on seal formation was significantly and in contrast wetting rate was irrelevant. The mechanisms describing the effect of soil texture, wetting rate, ESP and gypsum on IR and erosion are discussed.

INTRODUCTION

Understanding the soil degradation and structure stabilizing processes affected by irrigation water quality is essential for better management of soils and water resources. In Israel the use of EW for irrigation with the total salt concentration 15-20 mmolc L-1, may increase in the sodium adsorption ratio (SAR) from 2 in FW to 5-10 in the EW (Feigin et al., 1991). It may leads to a similar increase in the exchangeable sodium percentage (ESP) of the soil, which can significantly increase a soil's susceptibility to reduced permeability, seal formation and erosion. This is particularly substantive during the rainy season when the soil is exposed to water containing low levels of electrolytes (Shainberg and Letey, 1984). Presence of dissolved organic matter and suspended solids in EW enhanced soil - clay dispersivity, increased clay flocculation value and was considered responsible for a decrease in the HC of soil (Tarchitzky et al., 1999). Seal formation in irrigated soils is due to (Agassi et al., 1981; McIntyre, 1958): (i) physical disintegration of soil aggregates and compaction; and (ii) a physicochemical dispersion and formation a layer of very low permeability. The first mechanism is determined basically by the kinetic energy (KE) of the drops and the stability of the soil aggregates, while the second is controlled by the concentration and composition of the cations in the soil and applied irrigation water. (Shainberg and Singer, 1988; Levy et al., 1994). For many soils, aggregate breakdown may result even without mechanical impact, easily by slaking and depend, in addition to sodicity and electrolyte concentration and prewetting rate (Kemper and Rosenau, 1984; Truman et al., 1990; Levy et al., 1997). Prewetting at low rate maintain high HC and IR and prevent the deterioration of the primary structure, where fast prewetting disintegrates the aggregates and impairs the hydraulic properties of soils. Improving aggregate stability and preventing clay dispersion can be obtained by applying gypsum. Surface spread gypsum, by releasing electrolytes at the surface to percolating and runoff waters, decreases soil erosion (Agassi et al., 1981). For the soils varying in its texture and irrigated with EW and FW the associated effect of rain energy, ESP, gypsum and rate of wetting on seal formation and soil loss has been studied in lesser detail and were objectives of this study.

MATERIAL and METHODS

Samples of five smectitic soils (0-25 cm) irrigated FW and treated EW was used (Table 1).

HC was studied on soil columns, that were prepared by packing 120 g of air-dried (< 2 mm) soil into small cylinders ( Ų 54 mm with sand at the bottom). The fast wetting (50 mm h-1) was done by saturating the soil (0.5 M chloride solution with SAR 0, 6, 10, 15) via a Mariotte bottle placed at the bottom of the column. During leaching with same solution, the leachates were collected in tubes with a fraction collector, its volume were measured. IR, runoff and interrill soil loss were studied using a drip-type rainfall simulator at two rain KE (8 and 16 kJ m-3 ) with intensity 36 mm h-1. Air-dried aggregates (<4 mm) were packed in 200´ 400 mm trays, 20 mm deep. The trays were wetted with tap water from bottom by peristaltic pump at the rate of 8 (slow) and 64 (fast) mm h-1 and then were exposed to 60 mm of distilled water rainstorm at a slope of 15%. Infiltrated water, runoff and sediments were collected regularly. On fast prewetted soil phosphogypsum (PG) spread over the soil surface prior to rainstorm with high KE. For quantitative comparison between treatments, four parameters were used: (i) relative hydraulic conductivity (RHC) of soils; (ii) the final IR at the end of the storm; (iii) the total runoff (TR); (iv) the soil loss.

RESULTS and DISCUSSION

Effect of water quality on HC : The RHC of the soils as a function of effluent volume decreased with leaching (Figure 1). The difference in the RHC between the soil types can be explained by the difference in their texture. The higher the clay contents, ESP of the sample and SAR of solution, the lower were the RHC. For a given soil it was lower in EW irrigated samples. The disruptive forces associated with the fast wetting process that caused slaking of the soil, leading to the disintegration of the aggregates into microaggregates and primary particles. Differential swelling due to entrapped air was also accounted and was higher for a soil with high clay content. In EW irrigated samples (as excepted), the processes for HC deterioration as related to the ESP and/or SAR, physico-chemical mechanism and physical disintegration were more considerable.

Effect of rain energy and gypsum : For a given soil and type of irrigation water (FW and EW) an increase in raindrop KE resulted in (i) the lower final IR; (i) the higher runoff and soil loss. In fast prewetted samples when the low rain KE (8 kJ m-3) was used, final IR values for FW irrigated samples were practically higher (>2 mm h-1) than those for EW irrigated ones. However under high rain KE (16 kJ m-3) the surface aggregates of soils showed analogous weak resistance to the raindrops impact and as a result, the difference in IR values of soil' seal were (< 1 mm h-1) insignificant (Figure 2). Total runoff and soil loss values increased with an increase in rain KE, but unlike the runoff and final IR, soil loss values of the FW irrigated samples were significantly lower (150-400 g m-2) than those of the EW samples in high rain KE too (Figure 3). Mostly notable differences (> 2.5 mm) were noted among the total runoff values (which represent the changes in the IR curve through the entire rainstorm) of soil types. In the FW irrigated samples, the soil loss of the unstable loess, were higher than those of the grumusols, however in EW irrigated samples there were noticed opposite results due to combined effect of ESP and clay content on seal formation.

Smectitic soils are more erodible because they are more dispersive and formed a less permeable seal that generated higher levels of runoff and soil loss. Seal formation were more susceptible to the quality of irrigation water at low rain KE (as an effect of physico-chemical mechanism of seal formation) and were less sensitive at high rain KE, where the progressive breakdown of aggregates due to drop impact is very pronounced (Agassi et. al., 1981). Cultivation renders soils with a weaker structural stability and breakdown of aggregates by the impact of high rain KE played a dominant role in seal formation. Furthermore high prewetting rate of the surface aggregates was evidently rapid, and led to soils' aggregates slaking due to compressed entrapped air and/or swelling. In the presence of gypsum clay dispersion is prevented and seals formed results predominantly from drop impact. Thus, seal formation, soil loss in sandy loam (hamra) and sandy clay loam (loess) was more susceptible to the both raindrop KE and gypsum treatment.

The differences in the IR curves between FW and EW irrigated samples were also the result of chemical dispersion caused by soil sodicity. When 5 t ha-1 of PG applied to the soil the rate of drop in the IR became more gradual compared with the non-treated soils and the high final IR values (> 10 mm-1) maintained (Figure 2). PG was effective in controlling runoff and soil loss and reduced its values > 30% of that obtained in untreated soil. PG prevents clay dispersion and results in bigger aggregates at the soil surface and this in turn, is less susceptible to erosion by runoff flow (Kazman et al., 1983). Effect of gypsum in absolute term was more pronounced in the more erodible soils, such as EW irrigated soils, especially in hamra and loess. This fact is attributed to the susceptibility of the unstable soils to chemical clay dispersion, which in the PG amended soils was diminished. However final IR, runoff and soil loss of gypsum amended FW irrigated samples were considerably higher than EW irrigated samples. Efficiency of PG in soils irrigated with FW, indicate that some chemical dispersion took place even ESP < 2%. Under less erosive conditions, in FW irrigated stable soils, PG efficacy in controlling runoff and erosion are less exciting, though still important.

Effect of Prewetting Rate and Soil Texture : Prewetting rate prior to rainfall do influence on hydraulic properties of the soils and physical processes that control these indexes (Figure 3). For both rain KE and water quality (FW and EW), the final IR of soils are increased and runoff and soil loss values are decreased with (i) increasing in clay content; and (ii) decreasing in prewetting rate. The effect of prewetting rate was inconsequential (but still notable) in soils with clay content <20% and was very pronounced in soils with higher clay content (>50%). Degreasing in the prewetting rate increased the final IR (absolute difference 0.2-2.4 mm h-1 in unstable soils and 1.5-8.0 mm h-1 in stable soils), decreased runoff (2-12 and 12-20 mm) and soil loss (40-300 and 70-850 g m-2). When slow prewetting rate was used, the differences in the values of final IR, runoff and soil loss between FW and WW irrigated samples was still significant in soils with lower clay content (< 40%). However this difference was not substantial in clay soils, suggesting that the slow prewetting rate diminish the adverse dispersive effect of soil ESP on aggregate resistance to breaking down. The higher susceptibility of the hamra, loess and grumusols (H) to irrigation with EW was attributed to their lower clay content. The higher the clay contents, the more stable the aggregates, and thus the higher the resistance of the soil to seal formation (Ben-Hur et al., 1985).

In fast prewetted samples absolute difference in soil loss between FW and WW irrigated samples were in the range (90-260 g m-2) and (70-380 g m-2) in low and high rain KE respectively, whereas in slow prewetted soils it was found in the range (5-200 g m-2). In the presence of gypsum clay dispersion was prevented and seals formed results predominantly from drop impact. Consequently the process of seal formation was more sufficient on less stable soils. As a result decrease in runoff and erosion were more pronounced in gypsum treated soils. The role of gypsum on detachment of clay soil by raindrop impact is relatively small compared with wetting rate. Slow prewetting rate significantly reduce the effect of water quality on stable aggregated grumusol (Y) and (E) samples and thus hydraulic properties and erosion in these soils were more sensitive to wetting rate than to ESP level and PG treatment (Figure 4 and 5). Increased aggregate stability of slow prewetted soil is certainly due to the reductions of the volume of air that is entrapped during wetting. In both irrigated samples like to runoff, soil loss increase in clay content to a maximum at 20 % clay content and a further increase in clay content results a decrease in soil loss. (Ben-Hur et al., 1985; Levy et al., 1994).

The consequences of seal formation on soil loss are not simple. The decrease in IR due to sealing increases runoff, which may lead to increased erosion. Nevertheless, the detachability of seal is often lower than the detachability of the soil, so that seal formation decreased soil loss. The aggregate size distribution plays a major role in sealing because it partly controls the breakdown of the aggregates as well as the redistribution of the detached primary particles and aggregate fragments. A fragments resulting from slaking increase in size with increasing clay content. Decreasing prewetting rate further, diminish seal development that manifested itself in higher final IR, lower runoff levels and soil loss. The slow prewetting rate, gypsum decreases the amount of transportable particles and as a result detachment of soil aggregates and transport capacity of runoff are decreased (Truman and Bradford, 1990; Le Bissonnais and Singer, 1992; Levy et. al., 1997).

CONCLUSION

In the arid and semi arid zones more attention is now given to the environmental effects of irrigation with EW on cultivated soil surface degradation which favours runoff and erosion and thus increases pollution hazards by water-soluble and organic load contaminants. More interactions should occur between soil structure deterioration, sealing and erosion studies in order to determine soils suitability for EW irrigation and thus for evaluation of the land resource. The observed differences in the values of final IR, runoff and soil loss resulted from differences in the stability of the aggregates of these soils to wetting rate and aggregates' resistance to disintegration by the impact of the raindrops. PG application can be an efficient method in unstable soils where sealing mainly depends on raindrop energy and water quality. In stable soils sealing is mainly related to slaking by entrapped air compression, and impeding this processes is possible by controlling the wetting rate and little amount of PG application, which can be achieved in irrigated systems.

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