Runoff estimation in southern Brazil based on Smith's modified model and the Curve Number method

The objective of this work was to measure and model the runoff for different soils classes at different rainfall intensities (30, 60 and 120 mm h⁻¹) in Southern Brazil. A portable rainfall simulator with multiple nozzles was used to simulate these rainfall intensities. For each soil, the initial time and runoff rate, rainfall characteristics (total, duration and intensities), surface slope, crop residue amount and cover percentage, soil densities (bulk and particle), soil porosity (bulk, macro and micro), textural fractions (clay, silt and sand), and the initial and saturated soil water content were measured. The runoff measured was compared to Smith's modified and Curve Number (USDA-SCS) models. The cumulative runoff losses were 67, 45 and 27% of the total rainfall, for a Rhodic Paleudalf, Typic Quartzipsamment and Rhodic Hapludox, respectively. An inverse relationship was observed between initial runoff and the runoff rate, independently of the soil surface and rainfall conditions. Increasing rainfall intensity decreased the time to runoff and increased runoff rate. The Smith's modified model overestimated the cumulative runoff by about 4%. The Smith's modified model presented a better estimate for both higher and lower rainfall intensities (120 and 30 mm h⁻¹). The SCS Curve Number model overestimated the cumulative runoff by about 34%. This large overestimate is probably due to that the model did not take into account the soil tillage system used in the field by farmers, particularly for irrigated conditions. The combination of high porosity, low bulk density and presence of crop residue on soil surface decreased runoff losses, independently of the soil texture class. Smith's modified model better estimated the surface runoff for soil with a high soil water content, and it was considered satisfactory for Southern Brazil runoff estimations. The SCS Curve Number model overestimated the cumulative runoff and its use needs adjustments particularly for no-tillage management system.     

Soluble potassium transport in agricultural runoff water

Potassium transport in runoff from three agricultural soils was investigated in laboratory and field experiments using a kinetic equation describing soil K desorption. In the laboratory, this equation was used to study the effect of rainfall intensity and soil slope, cover, and residue incorporation on the effective depth of interaction between surface soil and runoff (edi), an important parameter in the kinetic equation. Using simulated rainfall, edi increased linearly (from 1.6 to 22.0 mm) with an increase in rainfall intensity (50 to 160 mm h⁻¹) and soil slope (2 to 20%). edi was reduced an average 82% following incorporation of 5.0 t ha⁻¹ of wheat straw (Triticum aestivum L. sp.) and 44% by a 0.5-mm² mesh screen, simulating crop cover, compared to the control (4.5 mm). This reduction was attributed to a decreased turbulent mixing of water at the soil surface brought about by an increased physical protection of soil. For all soils and treatments, edi was logarithmically related to soil loss (r² = 0.80), allowing estimation of edi under variable rainfall intensity, soil slope, type, and cover conditions from measured or estimated soil loss. The values of edi, thus estimated were used in the kinetic equation to predict solution K transport in runoff from eight agricultural watersheds in Oklahoma, U.S.A. Measured and predicted mean annual flow-weighted concentrations of solution K were not significantly different and were strongly correlated (r² = 0.92). These results improve our capability to predict K transport in runoff under field conditions. These methods could also be applied to other agricultural chemicals transported in runoff and results used to improve management of soil and water.     

Trickle irrigation of cotton: Effect on soil chemical properties

The management of soil salinity and sodicity in the root zone (0–150 cm) of Panoche clay loam soil was studied during three consecutive growing seasons in a field experiment designed to determine the water requirement of Acala SJ-2 cotton (Gossypium hirsutum L.) under trickle irrigation in the western San Joaquin Valley of California. The trickle irrigation treatments (20, 40, 60, 80 and 100% of the previous day's pan evaporation, PE) were imposed on each of three preplant furrow irrigation depths of 0, 190 and 380 mm. The electrical conductivity, EC, of the irrigation water ranged from 0.4 to 0.5 dS m⁻¹ unless groundwater had to be mixed with the surface supply; this practice raised the EC to 0.6–0.9 dS m⁻¹ for short time periods. The unadjusted sodium adsorption ratio, SAR, of the irrigation water averaged < 4.Soil salinity, as measured by EC_e, was < 5 dS m⁻¹ and sodicity, as measured by SAR_e, was < 15 in the root zone. Changes in these soil chemical properties were more related to the amount of preplant irrigation at the lower (< 40% PE) trickle-irrigation levels than at the higher levels. Results suggest that soil salinity and sodicity can be maintained at acceptably low levels by appropriate preplant irrigation with consideration to amount of winter rainfall; even when during the season only sufficient trickle irrigation is given to meet crop water requirements without regards to leaching needs. The lint cotton yields over the 3-year period ranged from 357 to 1542 kg ha⁻¹; the corresponding applied water ranged from 175 to 744 mm.     

Effects of water application intensity, drop size and water application amount on the characteristics of topsoil pores under sprinkler irrigation

Field experiments were carried out to study the effects of sprinkler irrigation on the characteristics of topsoil pores. Total soil porosity, capillary porosity, air-space porosity and porosities of different pore shapes were analyzed using images analysis of thin sections of soil samples. The experimental treatments included five water application intensities (5.3, 7.7, 11.0, 15.0 and 20.7 mm/h), five drop diameters (0.76, 1.28, 1.92, 3.18, 4.19 mm) and five water application amounts (9.0, 23.6, 37.5, 49.6, 59.4 mm). The compounding sprinkler system was used in the experiments of water application intensity and single sprinkler was used in the experiments of drop size and water application amount. The total porosity, air-space porosity and porosities of elongated pores have the similar decreasing tendency and pattern when water application intensity, drop diameter or water application amount increase. Capillary porosity, porosities of round and irregular pores have no obvious changing tendency. The decreasing porosities of the elongated pores and macropores are the main reasons for the decreasing of total porosity under sprinkler irrigation. To maintain soil structure in good conditions, the reasonable parameters would be considered for water application intensity, drop diameter and water application amount.     

Nitrogen leaching during sprinkler irrigation of a Dutch clay soil

Short-circuiting, which is vertical movement of free water through large continuous pores in an unsaturated soil matrix, was measured in the field in large columns from a cracked Dutch clay soil. The columns had been fertilized with chemical nitrogen fertilizer at a rate of 80 kg N ha^?1. Sprinkler irrigation (with an average intensity of 18 mm h^?1 and applied quantities varying from 8 mm to 22 mm) resulted in strong nitrogen leaching from the columns. Losses, which averaged 30%, resulted from mass-flow due to short-circuiting. Redox measurements suggested that no denitrification occurred. Application of only 6 mm of water slightly reduced nitrogen losses to approximately 15%. Losses could be reduced to 8% by applying the fertilizer to a wet soil surface which had just been sprinkled, and by sprinkling again the next day. In that way, the fertilizer grains dissolve and nitrogen diffuses into the surface soil, allowing less nitrogen movement along the soil surface towards the vertical cracks during the next sprinkling.     

Rainfall simulation to identify the storm-scale mechanisms of gully bank retreat

Gully erosion is one of the main causes of soil loss in drylands. Understanding the dominant mechanisms of erosion is important to achieve effective erosion control, thus in this study our main objective was to quantify the mechanisms involved in gully bank retreat as a result of three processes, falling of entire soil aggregates, transport of soil material by splash and by water running along gully banks (runoff), during rainfall events. The study was conducted in the sloping lands of the KwaZulu-Natal province, a region that is highly affected by gully erosion. Artificial rain was applied at 60 mm h⁻¹ for 45 min at the vertical wall of a gully bank typical to the area. The splash material was collected by using a network of 0.045 m² buckets. The sediments in the running water were assessed by sampling the runoff collected from a microplot inserted within the base of the bank, and collecting the fallen aggregates after the rainfall simulation was complete. Results indicated that the overall erosion for the simulation was 721 g m⁻² h⁻¹. Runoff erosion proved to be the dominant mechanism and amounted to 450 g m⁻² h⁻¹, followed by splash and fall down of aggregates (about 170 g m⁻² h⁻¹). Gully bank retreat occurred at a rate of 0.55 mm h⁻¹ and assuming that the soil bulk density is 1.3 g cm⁻³, this corresponds to a retreat of 8.8 mm y⁻¹. Extrapolations to the watershed level, where about 500 m² of gully bank are observed per hectare, would lead to an erosion rate of 4.8 t ha⁻¹ y⁻¹. These limited results based on a simulated storm show that the three main mechanisms (runoff, splash and fall down of aggregates) are responsible for the retreat of gully banks and that to mitigate gully erosion, appropriate measures are required to control all three mechanisms. Further research studies are needed to confirm and to scale up, both in time and space, as these data are obtained at one location and from a single artificial storm.     

Seasonal variations in nitrate leaching in structured clay soils under mixed land use

Nitrate leaching was studied for 2 years in a structured clay soil (Evesham series) under grass, winter wheat and spring barley at N fertilizer inputs of 135–144 kg ha^?1 year^?1. Measurements of soil water to 2 m depth by neutron probe showed that the year could be divided into well defined periods of deficit, separated by a period when the soil was at its winter mean water content. Soil water potentials showed very little gradient for water flow below 1 m, and a persistent convergent zero flux plane at 40–60 cm depth during the autumn wetting-up period (September—November).Nitrate concentration in the drainage increased with discharge rates up to 3–6 mm day^?1. Mean nitrate concentrations were generally highest during intermittent drain-flow in the autumn. Of the total N leached over the 2 years, 23 to 28% (5–7 kg N ha^?1) was lost during this period. The remainder (13–25 kg N ha^?1) was leached during winter and virtually no N was lost in the following spring-early summer. This seasonal pattern of N leaching was interpreted in terms of intermittent flow during rainfall of nitrate-rich water from surface layers, which bypassed the relatively dry soil matrix at 40–60 cm, but was intercepted by natural and artificial drainage channels. Implications for the prediction of N leaching loss based on the concept of excess winter rainfall are discussed. When predicting the start of N leaching in structured clay soils, the soil water status should be assessed from measurements of water potential rather than water content.     

Soil-cement tiles for lining irrigation canals

Laboratory flume test was conducted to investigate the effect of flowing water an soil-cement canal tiles. For this purpose, soil-cement tiles were constructed from different soils at various cement contents. A flume, 3 metre long and 100 mm wide, was lined with the tiles and the lined bed was subjected to flow velocities of around 2 m/s for a period of 7 days. The tiles made from coarse-textured soil (sandy loam and silt loam) aggregates of 5 mm and from fine textured soil (clay loam) aggregates of 2 mm size were found to be intact and smooth even when constructed at a cement contents lower than that needed to meet the durability requirements.Attempts were also made to measure seepage losses of soil-cement tile linings. A channel section of approximately 1 metre length with a side slope of 1:1 was constructed in the laboratory with the tiles and seepage losses measured by the ponding method were found to be in the range of 0.00123–0.00343 m³/m²/day.The results clearly suggest that soil-cement tiles (irrespective of type of soil) made with 2 mm or less size of soil aggregates are erosion resistant and due to very little or negligible rates of seepage losses, the soil-cement tile lining of irrigation canals is expected to be very promising especially in the areas where irrigation water is costly.     

保水剂对土壤特性长效影响的研究

基于田间试验,研究了保水剂对土壤理化性质的长效性影响。结果表明,施用保水剂可明显增加土壤微团粒结构,增大土壤持水率和表层土壤pH。保水剂施用1～5年期间,表层土壤(0～10cm)中0.05～0.01mm粒径质量分数增加了0.81%～7.74%,0.01～0.005mm粒径质量分数减少;施用保水剂第3年时表层土壤0.05～0.01mm粒径质量分数提高最显著,且保水剂施用后第3年土壤的控水能力最强。施用保水剂0～10、10～20、20～40cm土层土壤持水率比对照增加6.54%、6.22%、8.50%,饱和含水率分别比对照增加了1.3%～30.7%、2.4%～28.7%和0.8%～21.7%,保水剂连续施用第3年性能最佳。     

Functioning of mole drains in a clay soil

Observations of water-table fluctuation and drainflow are reported from a field experiment on a heavy clay soil with replicated “mole-drained” and undrained plots. Results from rainfall events indicated that in both treatments the dominant water movement was through the topsoil which in the drained plots was directly linked to the mole channels probably by fissures.     

Effects of dispersible soil clay and algae on seepage prevention from small dams

A field experiment was conducted on a red-brown earth (Natrixeralf) to find the effectiveness of spontaneously dispersed clay from sodic soils and mechanically dispersed clay (by puddling) from calcic and sodic soils in reducing the seepage loss of water from a series of small dams (pits). The effect of inoculating algae in the pits on reducing seepage was also investigated. A plastic lined pit was used for water balance control to measure incoming rainfall and evaporative loss.The results showed the effectiveness of dispersed soil clay in sealing the surface soil materials in the banks and beds of the pits. The dispersed clays from sodic soils were very effective in reducing the seepage to zero. When the clay concentration was above 8 g L⁻¹ the sealing was complete, irrespective of spontaneous or mechanical dispersion from sodic soils. The mechanically dispersed clay from calcic soils were less effective in sealing because of the deposition of flocculated materials in the pore systems formed domains and generated microporosity. In calcic pits, the inoculation of algae reduced the seepage by 13 to 23% and increase in biopolymer (chlorophyll and polysaccharide) production was only small.     

Determining percolation losses of packed clay soil from tensiometer data

The measurement or prediction of percolation losses in field situations is of great practical significance for efficient irrigation and for determination of the leaching requirement, particularly of clayey soils where impeded percolation occurs. Hydraulic properties and water losses in packed Ashutia clay soil were determined under prevented-evaporation and free-evaporation conditions using lysimeter and tensiometric techniques. Hydraulic conductivity was determined as a function of soil moisture content using percolation flux computed. An exponential relationship between hydraulic conductivity and soil water content K = ae^bθ, was found. The percolation and evaporation-plus-percolation fluxes estimated from tensiometer readings under prevented-and free-evaporation conditions, respectively, matched with profile water losses from lysimeter measurements. The error ranged between 0.01 and 0.82 mm day⁻¹ with high correlation coefficient indicating that water loss from a soil profile can be estimated from tensiometer readings.     

A new look at an old practice: Benefits from soil water accumulation in long fallows under Mediterranean conditions

The practice of long fallowing, by omitting a year of cropping, is gaining renewed focus in the low rainfall zone of the northern agriculture region of Western Australia. The impetus behind this practice change has been a reduced use of pasture breaks in cereal crop rotations, and the belief that a fallow can improve soil water accumulation and thus buffer the negative effects of dry seasons on crop yields. We evaluated the benefits of long fallowing (full stubble retention, no weed growth allowed) in a continuous wheat sequence via simulation modelling with APSIM at two rainfall locations and five soil types. The simulated benefits to long fallowing were attributable to soil water accumulation only, as the effects on soil nitrogen, diseases or weeds were not evaluated.The long-term (100 years) mean wheat yield benefit to fallowing was 0.36-0.43 t/ha in clay, 0.20-0.23 t/ha in sand and loam, and 0-0.03 t/ha in shallow sand and shallow loams. Over the range of seasons simulated the response varied from −0.20 to 3.87 t/ha in the clay and −0.48 to 2.0 t/ha for the other soils. The accumulation of soil water and associated yield benefits occurred in 30-40% of years on better soils and only 10-20% on poorer soils. For the loam soil, the majority of the yield increases occurred when the growing-season (May-September) rainfall following the fallow was low (<210 mm) and the difference in plant available soil water at sowing between fallowed and continuously cropped soil was high (>30 mm), although yield increase did occur with other combinations of growing-season rainfall and soil water. Over several years of a crop sequence involving fallow and wheat, the benefits from long fallowing due to greater soil water accumulation did not offset yield lost from omitting years from crop production, although the coefficient of variation for inter-annual farm grain production was reduced, particularly on clay soils during the 1998-2007 decade of below-average rainfall. We conclude that under future drying climates in Western Australia, fallowing may have a role to play in buffering the effects of enhanced inter-annual variability in rainfall. Investigations are required on the management of fallows, and management of subsequent crops (i.e. sowing earlier and crop density) so as to maximise yield benefits to subsequent crops while maintaining groundcover to prevent soil erosion.     

Determining water consumption in olive orchards using the water balance approach

Efficient irrigation regimes are becoming increasingly important in commercial orchards. Accurate measurements of the components of the water balance equation in olive orchards are required for optimising water management and for validating models related to the water balance in orchards and to crop water consumption. The aim of this work was to determine the components of the water balance in an olive orchard with mature ‘Manzanilla’ olive trees under three water treatments: treatment I, trees irrigated daily to supply crop water demand; treatment D, trees irrigated three times during the dry season, receiving a total of about 30% of the irrigation amount in treatment I; and treatment R, rainfed trees. The relationships between soil water content and soil hydraulic conductivity and between soil water content and soil matric potential were determined at different depths in situ at different locations in the orchard in order to estimate the rate of water lost by drainage. The average size and shape of the wet bulb under the dripper was simulated using the Philip’s theory. The results were validated for a 3 l h⁻¹ dripper in the orchard. The water amounts supplied to the I trees during the irrigation seasons of 1997 and 1998 were calculated based on the actual rainfall, the potential evapotranspiration in the area and the reduction coefficients determined previously for the particular orchard conditions. The calculated irrigation needs were 418 mm in 1997 and 389 mm in 1998. With these water supplies, the values of soil water content in the wet bulbs remained constant during the two dry seasons. The water losses by drainage estimated for the irrigation periods of 1997 and 1998 were 61 and 51 mm, respectively. These low values of water loss indicate that the irrigation amounts applied were adequate. For the hydrological year 1997–1998, the crop evapotranspiration was 653 mm in treatment I, 405 mm in treatment D and 378 mm in treatment R. Water losses by drainage were 119 mm in treatment I, 81 mm in treatment D and 4 mm in treatment R. The estimated water runoff was 345 mm in treatments I and R, and 348 mm in treatment D. These high values were due to heavy rainfall recorded in winter. The total rainfall during the hydrological year was 730 mm, about 1.4 times the average in the area. The simulated dimensions of the wet bulb given by the model based on the Philip’s theory showed a good agreement with the values measured. In a period in which the reference evapotranspiration was 7.9 mm per day, estimations of tree transpiration from sap flow measurements, and of evaporation from the soil surface from a relationship obtained for the orchard conditions, yielded an average daily evapotranspiration of 70 l for one I tree, and 48 l for one R tree.     

Latent heat flux over a furrow-irrigated tomato crop using Penman–Monteith equation with a variable surface canopy resistance

The Penman–Monteith (P–M) model with a variable surface canopy resistance (r_c) was evaluated to estimate latent heat flux (LE) or crop evapotranspiration (ET) over a furrow-irrigated tomato crop under different soil water status and atmospheric conditions. The hourly values of r_c were computed as a function of environmental variables (air temperature, vapor pressure deficit, net radiation, and soil heat flux) and a normalized soil water factor (F), which varies between 0 (wilting point, θ_WP) and 1 (field capacity, θ_FC). The Food and Agricultural Organization (FAO-56) method was also evaluated to calculate daily ET based on the reference evapotranspiration, crop coefficient and water stress coefficient. The performance of the P–M model and FAO-56 method were evaluated using LE values obtained from the Bowen ratio system. On a 20 min time interval, the P–M model estimated daytime variation of LE with a standard error of the estimate (SEE) of 46 Wm⁻² and an absolute relative error (ARE) of 3.6%. Thus, daily performance of the P–M model was good under soil water content ranging from 118 to 83 mm (θ_FC and θ_WP being 125 and 69 mm, respectively) and LAI ranging from 1.3 to 3.0. For this validation period, the calculated values of r_c and F ranged between 20 and 114 s m⁻¹ and between 0.87 and 0.25, respectively. In this case, the P–M model was able to predict daily ET with a SEE of 0.44 mm h⁻¹ (1.1 MJ m⁻² d⁻¹) and an ARE of 3.9%. Furthermore, the FAO-PM model computed daily ET with SEE and ARE values of 1.1 mm h⁻¹ (2.8 MJ m⁻² d⁻¹) and 5.2%, respectively.     

Image processing-based study of soil porosity and its effect on water movement through Andosol intact columns

The soil pore network and marcoporosity are important factors affecting water and solute transport. The transfer of contaminants to water resources is of particular importance in the Valle de Bravo watershed as it provides 10% of the drinking water for the 20 million inhabitants of Mexico City. This watershed is composed mainly of Andosols with unique mineralogical and physical characteristics. Soil porosity is usually examined on thin sections, using various image analysis techniques. We propose a novel methodology combining image analysis and a displacement experiment to study relationships between soil structure and water tracer transport parameters. H₂¹⁸O displacement experiments were conducted through intact soil columns sampled at three depths from a representative cultivated Andosol profile. The soil structure and pore characteristics were obtained by image analysis on thin sections obtained from each column at the end of the displacement experiment. The total 2D porosity (for pores larger than 50 μm) varied from 80% of the total section area in the topsoil to around 60% in the subsoil. Tubular pores were the most abundant in the soil profile, but ploughing of the topsoil had destroyed sections of these pores and replaced them with packing pores. Water transport in the intact subsoil columns was always in physical non-equilibrium, showing the existence of preferential flow pathways. In the topsoil, one column out of three showed no preferential flow, demonstrating that soil ploughing also homogenised pore connections. Pore connectivity was larger in the ploughed topsoil than in their deeper soil horizon counterparts. Our methodology offers a 2D quantitative characterisation of the macroporous network at 50 μm resolution and the determination of water transport parameters on the same intact soil samples. 3D characterisation of soil porosity using X-ray computed tomography (CT) gives a better picture of pore connection but usually has lower spatial resolution and a larger cost.     

The role of supplemental irrigation and nitrogen in producing bread wheat in the highlands of Iran

The West Asia and North Africa (WANA) region, with a Mediterranean climate type, has an increasing deficit in cereal production, especially bread wheat. Rainfed cropping in the highlands of this region coincides with the severely cold winter with mostly, snow from November to April. Cereal yields, are low and variable mainly as a result of inadequate and erratic seasonal rainfall and associated management factors, such as late sowing (or late crop emergence). In an area where water is limited, small amounts of supplemental irrigation (SI) water can make up for the deficits in seasonal rain and produce satisfactory and sustainable yields. This field study (1999–2002) on a deep clay silty soil in north west of Iran was conducted with four SI levels (rainfed, 1/3, 2/3 and full irrigation requirements) combined with different N rates (0, 30, 60, 90 and 120 kg ha⁻¹) with one wheat variety (Sabalan). Yields of rainfed wheat varied with seasonal rainfall and its distribution. A delay in the crop emergence from October (SI treatment) to November (rainfed) consistently reduced yields. With irrigation, crop responses to nitrogen were generally significant up to 60 kg N ha⁻¹. An addition of only limited irrigation (1/3 of full irrigation) significantly increased yields and maximized water use efficiency (WUE). Use efficiency for water and N was greatly increased by SI. Under deficit irrigation, maximum WUE would be achieved when 60 kg N ha⁻¹ is combined with 1/3 of full SI. Early crop germination is essential to ensure adequate crop stand before the winter frost and to achieve high yield. Early emergence can be achieved by applying a small amount (40–50 mm) of SI after sowing. Thus, when limited SI is combined with appropriate management, wheat production can be substantially and consistently increased in this highland semi-arid zone.     

Soil solution and exchange complex response to repeated wetting–drying with modestly saline–sodic water

Coal bed natural gas (CBNG) extraction in the Powder River (PR) Basin of Wyoming and Montana produces modestly saline-sodic wastewater, which may have electrical conductivity (EC) and sodium adsorption ratios (SAR) exceeding accepted thresholds for irrigation (EC = 3 dS m⁻¹, SAR = 12 (mmol_c l⁻¹)^1/2. As an approach to managing large volumes of CBNG-produced water, treatment processes have been developed to adjust produced water salinity and sodicity to published irrigation guidelines and legislated in-stream standards. The objective of this laboratory study was to assess acute and chronic soil solution EC and SAR responses to various wetting regimes simulating repeated flood irrigation with treated CBNG product water, followed by single rainfall events. Fifty-four soil samples from irrigated fields in southeast Montana were subjected to simulated PR water or CBNG water treated to EC and SAR values accepted as thresholds for designation of saline × sodic water, in a single wetting event, five wetting–drying events, or five wetting–drying events, followed by leaching with distilled water. Resultant saturated paste extract EC (ECe) and SAR of soils having <33% clay did not differ from one another, but resulting ECe and SAR were all less than those for soil having >33% clay. Repeated wetting with PR water having EC of 1.56 dS m⁻¹ and SAR of 4.54 led to SAR <12, but brought ECe near 3 dS m⁻¹. Repeated wetting with water having salinity = 3.12 dS m⁻¹ and SAR = 13.09 led to ECe >3 dS m⁻¹ and SAR near 12. Subsequent inundation and drainage with distilled water, simulating rainfall-quality leaching, reduced ECe and SAR more often in coarse-textured, high salt content soils than in finer-textured, lower salt content soils. Decreases in ECe upon leaching with distilled water were of greater magnitude than corresponding decreases in SAR, reinforcing supposition of sodium-induced dispersion of fine-textured soils as a consequence of rainfall following irrigation with water having salinity and sodicity levels equal to previously published thresholds.     

Modified rainfall simulator infiltrometer for infiltration,runoff and erosion studies

The rainfall simulator infiltrometer, that offers the advantages of low cost and high transportability, is modified and its performance is evaluated through field measurements of infiltration, runoff and soil erosion. The modified instrument is calibrated for various rainfall intensities. Further, to overcome the drawbacks associated with the traditional drop size determination methods, a relatively new approach, viz. computer vision technique, is used to find the raindrop sizes at 60 mm/h and 100 mm/h intensities. Field tests show that the rotating drop producing chamber results in better simulation of the natural rainfall conditions for infiltration, runoff and soil erosion studies.     

Assessment and management of poor quality waters for crop production: A simulation model (SWAM)

A simulation model has been prepared for assessing water quality to judge its suitability for irrigation. When water is classified as poor quality water (saline/sodic/saline-sodic) utilizing standard norms for Indian agro-climatic conditions, the model determines the potential of the water for direct application. Further, it also evaluates management strategies based on conjunctive use of fresh and saline waters. For this purpose, the model requires water quality data, crop data, soil data and rules established in the expert system rule-base. Data are compiled in data files which can be updated. For conjunctive use of saline and fresh waters, an irrigation scheduling sub-model has been modified to include a soil salinization-desalinization module based on layer-wise equilibrium theory. The module was independently tested using field data. The model SWAM has been successfully tested using data from a number of field experiments. Sodic waters of 16.2 meq l⁻¹ residual sodium carbonate would require 2.73 t ha⁻¹ of gypsum for each 20 cm of water applied to the soil. Field observations usually attest to this requirement. Likewise, saline water of 16 dS m⁻¹, when applied to a wheat crop in conjunction with fresh water of 0.5 dS m⁻¹, would yield optimally in case two saline water irrigations are followed by one fresh water irrigation in a normal rainfall year with an initial soil salinity of 2.98 dS m⁻¹. Some more useful data sets are analyzed and compared with results from field experiments. In our opinion, the model which is based upon recent guidelines can be applied to the classification of waters and their management. The minor changes necessary to apply the model to other conditions can be easily carried out.