Journal of Irrigation and Drainage Engineering

Four years of weekly soil water data measured by neutron probe were analyzed to determine average daily, monthly, and seasonal drip-irrigated hybrid poplar water use. The plantation studied is located near Boardman, Oregon, on the Columbia River Plateau. Irrigation application data, weekly rainfall, and changes in soil water content permitted the construction of a soil water balance model to calculate weekly hybrid poplar water use. Drainage was estimated by calculating potential soil water drainage from the lower soil profile. Sites with the potential for significant drainage were removed from the analysis, so that all sites used in the analysis could be assumed to be at steady state. Crop coefficients were calculated using reference evapotranspiration estimates obtained from a nearby AGRIMET weather station. Crop curves were estimated using a fit-by-hand method similar to that outlined by the United Nations Food and Agriculture Organization. Plant water use estimates and crop curves are presented for one-, two- and three-year-old hybrid poplars. Water use estimates represent upper bound estimates relative to the accuracy of the measurements made.

Wheat and sorghum are important cereal crops next only to rice in India and several other Asian countries. Improved water management requires accurate scheduling of irrigation, which in turn requires an accurate measurement of daily crop evapotranspiration ET_c. Thus, the first objective of this study was to measure daily, weekly, and seasonal crop ET_c of wheat and sorghum directly from sensitive weighing-type lysimeters. Experiments were conducted in a set of two electronic weighing-type lysimeters, 2 × 2 m in surface area and 2-m deep, to measure the hourly evapotranspiration of wheat and sorghum from 1991 to 1995 at Karnal, India. The average daily ET_c of wheat varied from <1 mm/d in the early growing period to >4 mm/d at milking stage. The peak ET_c of wheat was 4.6 mm/d and it occurred 16 weeks after sowing at the reproductive growth stage when leaf area index was 3.5. The measured seasonal ET_c of wheat was 336 mm. In the case of sorghum, ET_c was 3 mm/d at the initial stage, achieved a peak value of 6 mm/d between 8 and 9 weeks after sowing and declined to 4 mm/d during the maturity stage. The measured seasonal ET_c of sorghum was 495 mm. Precise information on crop coefficients for estimating ET_c, which is required for regional scale irrigation planning, is lacking in developing countries. Therefore, the second objective of this paper was to develop crop coefficients K_c for wheat and sorghum from ET_c measurements and weather data. The estimated values of K_c for wheat by the Penman-Monteith method at the four crop growth stages (initial, crop development, reproductive, and maturity) were 0.5, 1.36, 1.24, and 0.42, respectively, and for sorghum the K_c values at the four growth stages were 0.53, 0.82, 1.24, and 0.85, respectively. In the case of these two crops, actual K_c values are significantly different from those suggested by the Food and Agricultural Organization of the United Nations indicating the need for generating these values at the local/regional level. The third objective of this paper was to investigate the relationship between standard Food and Agricultural Organization of the United Nations Penman-Monteith, and other ET_r methods.

Soil salinity is a very common problem in today's irrigated agriculture. High salinity levels adversely impact crop yields and reduce overall soil quality. The presence of a saline shallow water table can be a major contributor to this problem. The LEACHC version of LEACHM is one of the few numerical models that considers independent movement of individual ions along with their detailed chemistry. This model has apparently not previously been tested under saline shallow water table conditions. LEACHC was evaluated using both lysimeter and field data from the literature. The model performed reasonably well in simulating solute transport above a saline shallow water table. For both data sets used in model validation, less reactive ions (sodium and chloride) were predicted well while calcium concentrations were underpredicted. For the field data, the model predicted soil electrical conductivity (EC) profiles better than most of the individual ions. The water content profiles associated with the field data were also predicted quite well. Based on these results, LEACHC was selected as a simulation tool for evaluating the effects of management practices on salinity transport in crop root zones above a saline shallow water table.

Root zone salinity is one of the major factors adversely affecting crop production. A saline shallow water table can contribute significantly to salinity increases in the root zone. A soil salinity model (LEACHC) was used to simulate the effects of various management alternatives and initial conditions on root zone salinity, given a consistently high water table. The impact of water table salinity levels, irrigation management strategies, soil types, and crop types on the accumulation of salts in the root zone and on crop yields was evaluated. There were clear differences in soil salinity accumulations depending upon the depth and salinity of the water table. In general, increasing water table depth reduced average soil profile salinity, as did having lower salinity in the water table. Among the four irrigation strategies that were compared, the 14-day irrigation interval with replenishment of 75% of evapotranspiration (ET) resulted in the lowest soil salinity. With a 4-day interval and 50% ET replenishment, a wheat yield reduction of nearly 40% was predicted after three years of salt accumulation. Soil type and crop type had minimal or no impact on soil salinity accumulation. Under all conditions, soil water average electrical conductivity increased during the 3-year simulation period. This trend continued when the simulation period was extended to 6 years. Under the conditions shown to develop the highest average soil salinity (high water table, low irrigation), an annual presowing irrigation of 125 mm caused a nearly 50% reduction in soil salinity at the end of the 6-year simulation period, as compared with the soil salinity given no presowing irrigation.

Several different finite-element grid configurations were evaluated for use in the numerical approximation of steady and transient flow to a single drain. By comparing the numerically simulated drain flow rates and head distributions with analytic values, a nested configuration was found to be appropriate for an effective drain radius of 0.01 m, and a square configuration was suitable for an effective drain radius of 0.05 m. Using an analytic solution, a method was developed to determine the distance of influence of a drain as a function of its effective radius and the geometry of its flow domain. The distance of influence was found to be independent of material type. An appropriate between-drain grid spacing was selected for the numerical simulation of multiple drains by increasing the grid mesh spacing outside the distance of influence. The position of the water table and drain flow rate with time were used to evaluate the between-drain grid spacing for transient variably saturated flow. Grid Péclet and Courant numbers, together with the shape of the solute advance front, were used to evaluate the suitability of the selected single drain configuration and between-drain grid spacing for solute transport. The resulting finite-element grid configuration for single and multiple drains ensures a stable efficient numerical solution, and it has applicability to numerical modeling of multiple subsurface drains.

A numerical solution of two-dimensional free surface flow to ditch drains in homogeneous and anisotropic soils is presented. The differential equation governing two dimensional ground-water flow in anisotropic soils is solved by the alternating direction implicit (ADI) method of finite difference. In the approach the method is modified to make it applicable for curved boundaries. This simplified approach does not involve any kind of smoothing or linearization technique. The model solutions for ditch drainage with constant replenishment and instantaneous drawdown are found to be in close agreement with the finite-difference and finite-element solutions proposed by other researchers.

This paper presents an experimental investigation of the noncohesive sediment threshold under a unidirectional steady-uniform stream flow on streamwise sloping (down slope and adverse) sedimentary beds. The theoretical analysis of the equilibrium of a solitary sediment particle lying on a sloping sedimentary bed shows that the threshold shear stress ratio, that is, the ratio of threshold shear stress for sloping bed to that for plane bed, is a function of the streamwise bed slope and angle of repose of sediment particles. Experiments carried out in two long rectangular ducts (closed-conduit flow) with nine types of sediments show that the equation derived from theoretical analysis corresponds closely to the experimental data. In an open channel flow (laboratory flume study), uniform flow is a difficult, if not impossible, proposition for a steeply sloping channel and is impossible to obtain in an adversely sloping channel. To avoid this problem, the tests were conducted with a closed-conduit flow.

A procedure is introduced to estimate solar radiation and subsequently reference crop evapotranspiration using minimum climatological data. The paper describes a modification to an original equation that uses maximum and minimum temperature to estimate solar radiation and reference crop evapotranspiration. The proposed modification allows for the correction of errors associated with indirect climatological parameters affecting the local temperature range. The proposed modification also improves the accuracy of estimates of solar radiation from temperature.

This technical note presents the application of a Radial Basis Function (RBF) network to estimate the FAO Blaney-Criddle b factor. Tabular b values are given in the United Nations Food and Agriculture Organization Irrigation and Drainage Paper Number 24. The b values obtained by the RBF network are compared to the appropriate b values produced using regression equations. The RBF network predicted b values better than the regression equations. An example is given to illustrate the simplicity and accuracy of the RBF network for b factor estimation.

The standard step method is commonly used to compute free surface profiles in gradually varied flow (GVF) through open channels. In this study, generalized numerical solutions in the Chebyshev form are presented for the standard step method to compute the free surface profiles in GVF without using look-up tables, interpolation procedures, or simplified assumptions concerning the cross-section geometry. The solutions are obtained using the flow resistance equations of Manning, Chezy, and Colebrook-White. The necessary parameters of some particular cases, namely rectangular, triangular, trapezoidal, circular, and exponential channels, are furnished. The use of the Chebyshev approximation has the advantage of requiring less iteration than the Newton-Raphson approximation.