Cautionary notes on the use of pedotransfer functions for estimating soil hydraulic properties

The performance of published pedotransfer functions was evaluated in terms of predicted soil water content, pressure heads, and drainage fluxes for a layered profile. The pedotransfer functions developed by Vereecken et al. (1989), Vereecken et al. (1990) were used to determine parameters of the soil hydraulic functions θ(h) and K(h) which were then used as input to SWATRER, a transient one-dimensional finite difference soil water model with root uptake capability. The SWATRER model was used to simulate the hydraulic response of a multi-layered soil profile under natural climatic boundary conditions for a period of one year. The simulations were repeated by replacing the indirectly estimated water retention characteristic by (1) local-scale, and (2) field-scale mean observed θ(h) relationships. Soil moisture contents and pressure heads simulated at different depths in the soil profile were compared to measured values using these three different sets of hydraulic functions. Drainage fluxes at one meter below ground surface have also been simulated using the same three sets of hydraulic functions. Results show that simulations based on indirectly estimated moisture retention characteristics (obtained from pedotransfer functions) overpredict the observed moisture contents throughout the whole soil profile, but predict the pressure heads at shallow depths reasonably good. The results also show that the predicted drainage fluxes based on estimated retention functions are about four times as high compared to the drainage fluxes simulated using measured retention curves.     

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.     

Simulation of yield decline as a result of water stress with a robust soil water balance model

The relative yield decline that is expected under specific levels of water stress at different moments in the growing period is estimated by integrating the FAO K_y approach [Doorenbos, J., Kassam, A.H., 1979. Yield response to water. FAO Irrigation and Drainage Paper No. 33. Rome, Italy] in the soil water balance model BUDGET. The water stored in the root zone is determined in the soil water balance model on a daily basis by keeping track of incoming and outgoing water fluxes at its boundary. Given the simulated soil water content in the root zone, the corresponding crop water stress is determined. Subsequently, the yield decline is estimated with the K_y approach. In the K_y approach the relation between water stress in a particular growth stage and the corresponding expected yield is described by a linear function. To account for the effect of water stresses in the various growth stages, the multiplicative, seasonal and minimal approach are integrated in the model. To evaluate the model, the simulated yields for two crops under various levels of water stress in two different environments were compared with observed yields: winter wheat under three different water application levels in the North of Tunisia, and maize in three different farmers’ fields in different years in the South West of Burkina Faso. Simulated crop yields agreed well with observed yields for both locations using the multiplicative approach. The correlation value (R²) between observed and simulated yields ranged from 0.87 to 0.94 with very high modeling efficiencies. The root mean square error values are relatively small and ranged between 7 and 9%. The minimal and seasonal approaches performed significantly less accurately in both of the study areas. Estimation of yields on basis of relative transpiration performed significantly better than estimations on basis of relative evapotranspiration in Burkina Faso. A sensitivity analysis showed that the model is robust and that good estimates can be obtained in both regions even by using indicative values for the required crop and soil parameters. The minimal input requirement, the robustness of the model and its ability to describe the effect on seasonal yield of water stress occurring at particular moments in the growing period, make the model very useful for the design of deficit irrigation strategies. BUDGET is public domain software and hence freely available. An installation disk and manual can be downloaded from the web.     

An integrated model-approach to the effect of water management on crop yield

Quantifying the effect of drainage on crop yield is of essential importance in agricultural management. In this article a model is described with which this effect can be computed. For both arable land and grassland the factors acting in spring, summer and autumn are dealt with separately.Arable land. In spring sowing date is the main factor affecting the crop yield. Sowing date depends on the tillage conditions of the soil toplayer. By means of an existing model, the course in time of the soil water tension of the upper layer is simulated in connection with rainfall, evaporation, drain depth and drain intensity data. Using specific criteria on minimum soil water tension for tillage operations, the dates and number of workable days can be established from the model output. The expected yield depression is then derived, using an experimental relationship between yield depression and number of days of sowing delay.During the growing season the yield directly depends on the magnitude of the actual evapotranspiration. This value can be computed by means of a known evapotranspiration model for various drought frequencies, groundwater table depths in spring, drain intensities and amounts of water supplied. The yield can be obtained from the relationship between yield and relative evapotranspiration. Combining this yield with the yield depression obtained by means of the workability model gives the actual yield.In autumn crop yield is influenced by the working conditions during harvest. Via the workability model, the dates and the number of days available for harvesting are determined. Yields are derived from an experimental relationship between yield depression and number of days of earlier harvesting. An example is given for summer cereals growing on a heavy sandy loam soil under meteorological conditions prevailing in The Netherlands.Grassland. The effect of shallow groundwater table depths in winter and spring on the yield of the first and second cut can be determined with the workability model in an identical manner to that given for arable land. Because of lack of data, a slightly different approach was followed in this paper. With the workability model the course of groundwater table depth during winter and spring can be simulated and the mean depth determined. From the relation between yield depression and mean groundwater table depth over the period November through May the yield depression can be found. Combining this with the yield obtained with the evapotranspiration model gives the actual yield. An example representative for The Netherlands is given for grass on peat soil.     

Antecedent water content effects on runoff and sediment yields from two Coastal Plain Ultisols

The highly weathered, low-carbon, intensively cropped, drought-prone Coastal Plain soils of Georgia are susceptible to runoff and soil loss, especially at certain times of the year when soil water contents are elevated. We quantified the effects of antecedent water content (AWC) on runoff (R) and sediment (E) losses from two loamy sands managed under conventional- (CT), strip- (ST), and/or no-till (NT) systems. Two AWC treatments were evaluated: field moist (FM) and pre-wet (PW), created with and without post pesticide application irrigations (∼12 mm of water added with the rainfall simulated over 30 min) for incorporation. Treatments (5) evaluated were: CT + FM, CT + PW, ST + FM, ST + PW, and NT + PW. Field plots, each 2-m × -3 m, were established on each treatment. Each 6-m² field plot received simulated rainfall at a variable rainfall intensity (I_v) pattern for 70 min (site 1) or a constant rainfall intensity (I_c) pattern for 60 min (site 2; I_c = 50.8 mm h⁻¹). Adding ∼12 mm of water as herbicide incorporation increased AWCs of the 0-2 (3-9-fold) and 2-15 (23-117%) cm soil depths of PW plots compared to existing field moist soil conditions. Increase in AWC increased R (as much as 60%) and maximum R rates (as much as 62%), and decreased E (at least 59%) and maximum E rates (as much as 2.1-fold) for corresponding tillage treatments. Compared to CT plots, ST and NT plots decreased R (at least 2.6-fold) and maximum R rates (as much as 3-fold), and decreased E (at least 2.7-fold) and maximum E rates (at least 3.2-fold). Runoff curves for pre-wetted CT and ST plots were always higher than corresponding FM curves, whereas E curves for field moist CT and ST plots were always higher than corresponding PW curves. Changes in AWC and tillage affected detachment and transport processes controlling runoff and sediment yields. A more accurate measure of rainfall partitioning and detachment and transport processes affecting R and E losses was obtained when commonly occurring field conditions (increased AWC with irrigation; I_v pattern derived from natural rainfall; commonly used tillage systems) were created and evaluated.     

A scaling based estimation of soil unsaturated hydraulic properties at a field scale

This paper deals with the prediction of the soil water retention h(S) and the soil unsaturated hydraulic conductivity K(S) functions of a clay-loam soil at a field scale (1 ha) where the variable S represents water saturation. The Van Genuchten model and the corresponding Mualem-Van Genuchten model were used to predict h(S) and K(S) functions respectively. The field data (tensiometric and neutron probe measurements) used in this study were provided by the soil water balance (four neutron sites, 0.35 to 1.55 m soil layer) of a soybean crop over a 78 days growing season. The advantages of the scaling approach for describing the field variability of the h(S) function were confirmed. The scaling approach accounted for 73% of the field variability of the soil matrix potential. A simple procedure was proposed in order to predict the K(S) function using scaling theory. This was done by simultaneously applying a ``zero flux method' and ``deep flux method' to compute the soil water balance and fit the saturated hydraulic conductivitiy (K _sat), the only unknown parameter in K(S). Received: 15 November 1995     

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.     

Modelling for maize irrigation scheduling using long term experimental data from Plovdiv region, Bulgaria

Independent historic datasets on irrigated maize, collected over seven years (1984-1990), were used to parameterize the irrigation scheduling model ISAREG. Experimental data were obtained under rainfed, deficit, and full irrigation conditions in an alluvial soil at Tsalapitsa, Plovdiv region, in the Thracian plain, Bulgaria. Crop coefficients and depletion fractions for no-stress were calibrated by minimizing the differences between observed and simulated soil water content. The calibration was performed using data from full irrigation and rainfed treatments while deficit irrigation treatments were used for validation. The modelling efficiency was high, 0.91 for the calibration and 0.89 for the validation. The resulting average absolute errors of the estimate for the soil water content were smaller than 0.01 cm³ cm⁻³. The model was also tested by comparing computed versus observed seasonal evapotranspiration. Results for dry years show a modelling efficiency of 0.96 but the model slightly underestimated evapotranspiration for other years. The yield response factor was derived from observed yield data of the hybrid variety H708 when relative evapotranspiration deficits were smaller than 0.5. The value K_y = 1.32 was obtained. The relative yield decreases predicted with this K_y value compared well with observed data. Results support the use of the ISAREG model for developing water saving irrigation schedules for the Thracian plain.     

Agricultural water management in a humid region: sensitivity to climate,soil and crop parameters

A sensitivity analysis of irrigation water requirements at the regional scale was conducted for the humid southeastern United States. The GIS-based water resources and agricultural permitting and planning system (GWRAPPS), a regional scale, GIS-based, crop water requirement model, was used to simulate the effect of climate, soil, and crop parameters on crop irrigation requirements. The effects of reference evapotranspiration (ET_o) methods, available soil water holding capacities (ASWHC), crop coefficients (K_c), and crop root zone depths (z) were quantified for 203 ferneries and 152 potato farms. The irrigation demand exhibited a positive relationship with K_c and z, a negative relationship with ASWHC, and seasonal variations depending on the choice of ET_o methods. The average irrigation demand was most sensitive to the choice of K_c with a 10% shift in K_c values resulting in approximately 15% change in irrigation requirements. Most ET_o methods performed reasonably well in estimating annual irrigation requirements as compared to the FAO-56 PM method. However, large differences in monthly irrigation estimates were observed due to the effect of the seasonal variability exhibited by the methods. Our results suggested that the selection of ET_o method is more critical when modeling irrigation requirements at a shorter temporal scale (daily or monthly) as necessary for many applications, such as daily irrigation scheduling, than at a longer temporal scale (seasonal or annual). The irrigation requirements were more sensitive to z when the resultant timing of irrigation coincided with rainfall events. When compared with the overall average of the irrigation requirements differences, the site-to-site variability was low for K_c values and high for the other variables. In particular, soil properties had considerable average regional differences and variability among sites. Thus, the extrapolation of site-specific sensitivity studies may not be appropriate for the determination of regional responses crop water demand.     

Simulation of the dry matter production and seed yield of common beans under varying soil water and salinity conditions

We present a model that simulates the effects of water and salinity stress on the growth of beans. The model derives a combined soil water/salinity stress factor from the total water potential (combination of the matric and the osmotic potentials) and uses this stress factor as a growth limiter in a growth model. The model was tested on data obtained from two greenhouse trials of beans (Phaseolus vulgaris) grown under a range of soil water and salinity conditions. The simulated dry weight of the bean generally followed those observed. In the first trial, the comparison between simulated and observed total dry weight and seed yield gave R² values of 0.97 and 0.92, respectively. Comparison of the simulated to the observed dry weight for the second trial gave R² values of 0.85 and 0.89, respectively. These indicate a good performance of the model in general. The principle of deriving a combined water/salinity stress from the matric and osmotic potentials is simple and can be included as a simple routine in many existing crop models without much difficulty.     

Sugarcane transpiration with shallow water-table: sap flow measurements and modelling

The most common sugarcane variety in the Gharb plain of Morocco (CP 66-345 variety) was grown in a lysimeter in the laboratory. It developed during 6 months with a water-table at 0.7 m below the soil surface. The water-table was then successively maintained with a Mariotte bottle at 0.45, 0.2 and 0.05 m from the soil surface for 21, 31 and 24 days, respectively. Transpiration was measured by Dynamax sap flow sensors. Soil water pressure heads were measured at six different depths; soil hydraulic properties and root density profile were also determined. No transpiration reduction was observed with soil waterlogging. Two different models were used to predict the pattern of root water uptake (RWU) with water-table at 0.45 m below the soil surface. These two models are based on a RWU function used as sink term in the Richards equation. The first model, HYDRUS-2D (Simunek et al., 1996), is based on the α-model RWU (Feddes et al., 1978a) which depends on a reduction function varying according to the soil water pressure head and on the root density. The second model, SIC (Breitkopf and Touzot, 1992) is based on the h_r-model RWU (Whisler and Millington, 1968, Feddes et al., 1974). It is proportional to the difference between soil and root pressure heads, to unsaturated hydraulic conductivity and to root density. Calculated soil water flows from pressure head measurements are compared to predicted pressure heads by the two models. These predictions compare well with the measured values and show that sugarcane roots mainly absorbed water in the water-table. However, while goods predictions were obtained using the actual root density profile with the h_r-model, it was necessary to modify this profile to obtain proper results using the α-model.     

Simulating water content, crop yield and nitrate-N loss under free and controlled tile drainage with subsurface irrigation using the DSSAT model

In southwestern Ontario, rain-fed crop production frequently fails to achieve its yield potential because of growing-season droughts and/or uneven rainfall distribution. The objective of this study was to determine if the Decision Support System for Agrotechnology Transfer (DSSAT) v4.5 model could adequately simulate corn and soybean yields, near-surface soil water contents, and cumulative nitrate-N losses associated with regular free tile drainage (TD) and controlled tile drainage with optional subsurface irrigation (CDS). The simulations were compared to observations collected between 2000 and 2004 from both TD and CDS field experiments on a Perth clay loam soil at the Essex Region Conservation Authority demonstration farm, Holiday Beach, Ontario, Canada. There was good model-data agreement for crop yields, near-surface (0-30 cm) soil water content and cumulative annual tile nitrate-N loss in both the calibration and validation years. For both TD and CDS, the CENTURY soil C/N model in DSSAT simulated water content and cumulative tile nitrate-N loss with normalized root mean square error (n-RMSE) values ranging from 9.9 to 14.8% and 17.8 to 25.2%, respectively. The CERES-Maize and CROPGRO-Soybean crop system models in the DSSAT simulated corn and soybean yields with n-RMSE values ranging from 4.3 to 14.0%. It was concluded that the DSSAT v4.5 model can be a useful tool for simulating near-surface soil water content, cumulative tile nitrate-N losses, and corn and soybean yields associated with CDS and TD water management systems.     

An upscaling approach to simulate unsaturated flow in horizontally heterogeneous soils at field scale under flood irrigation

An upscaling approach was developed for simulating soil water flow in horizontal heterogeneous unsaturated zone at field scale under flood irrigation. Based on the assumption of stream tube model and the van Genuchten-Mualem soil hydraulic function with five parameters K_s, α, n, θ_r and θ_s, the Richards equation was transformed into a dimensionless form by using the dimensionless forms of temporal and spatial variables, and pressure head. Although a strong dependency of parameters K_s and α on scale does not exist in the transformed Richards equation, the parameter n, which is slightly dependent on scale, still exists in the transformed equation, and the parameter α is introduced in the transformed initial and boundary conditions. Therefore, a power law averaging technique was also included in our upscaling approach. Compared with traditional numerical methods, the distribution of pressure head of each soil column, and the mean and variance of soil water dynamics in all the soil columns can be obtained by numerically solving the transformed Richards equation only one time and by returning to the dimensionless variable expressions, while the traditional Richards equation must be solved once for every soil column. The new approach was calibrated by two numerical experiments, and the soil water content as a function of time and depth was reasonably well simulated for the two experiments which involved different soil textures. Different combinations of α and n were applied for comparing the accuracy of numerical simulations: n had little effect on the simulated results whereas α had some significant effect on the simulated results. To further verify the numerical efficiency of the new approach, we adopted the effective α values corresponding to the exponent p of the power law averaging technique [Eq. (16)] when p approaches 0, p = ± 1 and ± 0.5 to conduct more numerical experiments. It showed that with an initial pressure head profile of equilibrium for Example 1, and with an initial pressure head profile of constant for Example 2, the simulated results of the wetting front at the specific depths were in agreement with that of traditional numerical method, and the errors in simulated pressure heads were small. In summary, the proposed approach can be useful for upscaling unsaturated water flow characteristics in horizontally heterogeneous soils.     

Using the dual approach of FAO-56 for partitioning ET into soil and plant components for olive orchards in a semi-arid region

The main goal of this research was to evaluate the potential of the dual approach of FAO-56 for estimating actual crop evapotranspiration (AET) and its components (crop transpiration and soil evaporation) of an olive (Olea europaea L.) orchard in the semi-arid region of Tensift-basin (central of Morocco). Two years (2003 and 2004) of continuous measurements of AET with the eddy-covariance technique were used to test the performance of the model. The results showed that, by using the local values of basal crop coefficients, the approach simulates reasonably well AET over two growing seasons. The Root Mean Square Error (RMSE) between measured and simulated AET values during 2003 and 2004 were respectively about 0.54 and 0.71 mm per day. The basal crop coefficient (K_cb) value obtained for the olive orchard was similar in both seasons with an average of 0.54. This value was lower than that suggested by the FAO-56 (0.62). Similarly, the single approach of FAO-56 has been tested in the previous work (Er-Raki et al., 2008) over the same study site and it has been shown that this approach also simulates correctly AET when using the local crop coefficient and under no stress conditions.Since the dual approach predicts separately soil evaporation and plant transpiration, an attempt was made to compare the simulated components of AET with measurements obtained through a combination of eddy covariance and scaled-up sap flow measurements. The results showed that the model gives an acceptable estimate of plant transpiration and soil evaporation. The associated RMSE of plant transpiration and soil evaporation were 0.59 and 0.73 mm per day, respectively.Additionally, the irrigation efficiency was investigated by comparing the irrigation scheduling design used by the farmer to those recommended by the FAO model. It was found that although the amount of irrigation applied by the farmer (800 mm) during the growing season of olives was twice that recommended one by the FAO model (411 mm), the vegetation suffered from water stress during the summer. Such behaviour can be explained by inadequate distribution of irrigation. Consequently, the FAO model can be considered as a potentially useful tool for planning irrigation schedules on an operational basis.     

Movement of water in restructured saline and sodic clay topsoils under a rainfall simulator

The nature of water movement through freely draining saturated and field moist aggregates of saline sodic clay topsoil was studied using 200 mm long columns filled with soil aggregates. Water containing tritium as a tracer was supplied either by means of rainfall simulator or directly to the surface of the soil under a negative pressure head of 500 Pa.The proportion of macropore and micropore flow was elucidated. The micropores of the aggregates were shown to convey very little water (0.013 mm h⁻) and hence, even at low rainfall intensities water was expected to move down through the macropores. In practice, at a low water application rate of 0.6 mm h⁻ drainage did not begin from the base of the column until the aggregates had become fully saturated due to mobile water in the macropores being continuously absorbed into the micropores. The results, however, indicated that extensive rapid bypassing does occur at medium and high rainfall intensities ( > 2.3 mm ⁻) , with the result that a large proportion of the water falling on the unsaturated plough layers of clay soils is drained before the topsoil becomes saturated.The soil absorbed water continuously during the application of the equivalent of a wetter than average winter's rain (400 mm), the rate of absorption being directly proportional to the amount of salt leached.Tritium, used as a tracer, was found to be preferentially absorbed by the clay during the leaching process, the concentration in the soil water rising to 1.8 times that of the applied tritiated water.     

Development and evaluation of integrated water and nitrogen model for maize

Maize (Zea mays L.) is an important food crop for irrigated regions in the world. Its growth and production may be estimated by different crop models in which various relationships between growth and environmental parameters are used. For simulation of maize growth and grain yield, a simulation model was developed (Maize Simulation Model, MSM). Dynamic flow of water, nitrogen (N) movement, and heat flow through the soil were simulated in unsteady state conditions by numerical analysis in soil depth of 0–1.8 m. Hourly potential evapotranspiration [ET_p(t)] for maize field was estimated directly by Penman–Monteith method. Hourly potential evaporation [E_p(t)] was estimated based on ET_p(t) and canopy shadow projection. Actual evaporation of soil surface was estimated based on its potential value, relative humidity of air, water pressure head and temperature at soil surface layer. Actual transpiration (T_a(t)) was estimated based on soil water content and root distribution at each soil layer. Hourly N uptake by plant was simulated by N mass flow and diffusion processes. Hourly top dry matter production (HDMA^j + 1, where j is number of hours after planting) was estimated by hourly corrected intercepted radiation (RSLT^j + 1) by plant leaves [determined from leaf area index (LAI^j + 1)] with air temperature, the maximum and minimum plant top N concentration and the amounts of nitrogen uptake. The value of LAI^j + 1 at each hour was estimated by the accumulated top dry matter production at previous hour using an empirical equation. Maize grain yield was estimated by a relationship between harvest index and seasonal plant top dry matter production. The model was calibrated using data obtained under field conditions by a line source sprinkler irrigation. When the values of water and nitrogen application were optimum, grain yield (moisture content of 15.5%) was 16.2 Mg ha⁻¹. Model was validated using two independent experimental data obtained from other experiments in the Badjgah (Fars province). The experimental results validated the proposed simulation model fairly well.     

Crop rotations in Argentina: Analysis of water balance and yield using crop models

Cropping schemes have developed in east-central Argentina for rainfed soybean (Glycine max Merr.) production that invariably employ no-tillage management. Often these schemes include growing soybean in a sequence of crops including wheat (Triticum aestivum L.) and maize (Zea mays L.). The full impact of various rotation schemes on soil water balance through a sequence of seasons has not been explored, although the value of these rotations has been studied experimentally. The objective of this work was to investigate through simulations, potential differences in temporal soil water status among rotations over five years. In this study, mechanistic models of soybean (Soy), maize (Maz), and wheat (Wht) were linked over a five-years period at Marcos Juárez, Argentina to simulate soil water status, crop growth, and yield of four no-till rotations (Soy/Soy, Soy/Wht, Soy/Maz, and Soy/Maz/Wht). Published data on sowing dates and initial soil water contents in the first year from a no-till rotation experiment were used as inputs to the model. After the first year, soil water status output from the model was used to initiate the next crop simulation in the sequence. The results of these simulations indicated a positive impact on soil water balance resulting from crop residue on the soil surface under no-till management. Continuous soybean and the two-year soybean/maize rotation did not efficiently use the available water from rainfall. Residue from maize was simulated to be especially effective in suppressing soil evaporation. Thus, the Soy/Maz simulation results indicated that this rotation resulted in enhanced soil water retention, increased deep water percolation, and increased soybean yields compared with continuous soybean crops. The simulated results matched well with experimental observations. The three-crop rotation of Soy/Maz/Wht did not increase simulated soybean yields, but the additional water retained as a result of decreased soil evaporation resulting from the maize residue allowed the addition of a wheat crop in this two-year rotation. Simulated soybean yields were poorly correlated with both the amount of soil water at sowing and the rainfall during the cropping period. These results highlight the importance of temporal distribution of rainfall on final yield. These models proved a valuable tool for assessing the consequences of various rotation schemes now being employed in Argentina on temporal soil water status, and ultimately crop yield.     

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.     

Evaluation of irrigation schemes for sugarcane in Sindh,Pakistan, using SWAP93

A computer simulation model, SWAP93, was used to simulate the soil water balance of sugarcane (Saccharum officinarum L.) over a period of 6 years, in order to develop an efficient irrigation scheduling scheme for Sindh, Pakistan. Given the limitations and inflexibility of the existing warabandi irrigation system, which does not allow on-demand irrigation, only irrigation depth and irrigation interval were varied in order to assess the best irrigation depth/interval combination for sugarcane production. Twelve irrigation treatments were simulated. These treatments were four irrigation amounts (900, 1200, 1650 and 1800 mm) and three irrigation frequencies (7, 10 and 15 days). Three seasons with rainfall totaling less than 20 mm were compared with three seasons of over 200 mm rainfall. Two approaches were used in assessing the irrigation schemes: yield parameters and water management response indicators. Treatment parameters (e.g. irrigation amounts, weather conditions, soil characteristics, etc.) served as input for SWAP93, actual transpiration was calculated and then used in a crop water production function to predict yield and water use efficiency. Additionally, water management response indicators were derived from model outputs, and used to assess the impact of the schemes on soil salinity and water logging. Both these indicators and the yield and water use efficiency indicated that a seasonal total of 1650 mm, applied at a 15-day interval was the best irrigation scheme for the region.     

A distributed agro-hydrological model for irrigation water demand assessment

The actual irrigation water demand in a district in Sicily (Italy) was assessed by the spatially distributed agro-hydrological model SIMODIS (SImulation and Management of On-Demand Irrigation Systems). For each element with homogeneous crop and soil conditions, in which the considered area can be divided, the model numerically solves the one-dimensional water flow equation with vegetation parameters derived from Earth Observation data. In SIMODIS, the irrigation scheduling is set by means of two parameters: the threshold value of soil water pressure head in the root zone, h_m, and the fraction of soil water deficit to be re-filled, Δ. This study investigated the possibility of identifying a couple of irrigation parameters (h_m, Δ) which allowed to reproduce the actual irrigation water demand, given that the study area was adequately characterized with regard to the spatial distribution of the soil hydraulic properties and the vegetation conditions throughout the irrigation season. The spatial distribution of the soil and vegetation properties of the study area, covering an irrigation district of approximately 800 ha, was accurately characterized during the summer of 2002. The soil hydraulic properties were identified by an intensive undisturbed soil sampling, while the vegetation cover was characterized in terms of leaf area index, surface albedo and fractional soil cover by analysing multispectral LandSat TM imageries. Irrigation volumes were monitored at parcel scale.A reference scenario with h_m = −700 cm and Δ = 50% (corresponding to a mean actual to potential transpiration ratio of 0.95) allowed to reproduce the spatial and temporal distribution of the actual irrigation demand at the district scale. The spatial variability of the crop conditions in the considered area had much more influence to assess the irrigation water demand than the soil hydraulic spatial variability. The proposed approach showed that, under the agro-climatic conditions typical for the Mediterranean region, SIMODIS may be a valuable tool in managing irrigation to increase water productivity.