Journal of Irrigation and Drainage Engineering

The retirement of land from agricultural production as a means of reducing the volume of subsurface drainage generated in the Western San Joaquin Valley, Calif., is under consideration. Reticence among local farmers and water managers renders only willing-seller land retirement feasible. There is an interest to acquire land that will generate the maximum possible drainage reduction relative to a “no retirement” baseline. An investigation was conducted to determine the drainage reduction potential of the retirement of (1) parcels that span land underlain by tile drains and land free of drainage infrastructure; (2) a “downgradient” parcel plagued by shallow ground water and equipped with subsurface tile drains; and (3) an “upgradient” parcel overlying well-aerated soil requiring no drainage. Long-term modeling with a deforming finite-element model suggests that the contiguous retirement yields the greatest drainage reduction. For single parcels, the 31% drainage reduction potential of downgradient retirement appears more attractive than the 16% drainage reduction associated with upgradient retirement.

Semianalytical transient equations for shallow subsurface transverse drainage systems installed in sloping lands are developed. They provide a general relationship between drain flow rates, water table elevations, and recharge rates. This relationship demonstrates that, depending on the recharge intensity, several drain flow rates can be observed at a given water table elevation. The recharge contribution is shown to depend on a water table shape factor and to decrease when the water table is low or the slope is steep. For very steep slopes, the recharge intensity no longer influences the drain flow rate. These equations can be used to confirm previous results obtained in steady-state conditions and to determine precisely under which conditions slope needs to be considered in drainage design. They have been incorporated into the field drainage model SIDRA, which simulates hourly values of water table elevations and drain flow rates. The model predictions are compared with the predictions of a steady-state equation and a numerical model, which solves the Boussinesq equation (SLOP model).

Computer algebraic routines are applied for determination of the phreatic surface from standard boundary-value problems for ordinary differential equations. The method does not require iterative steps as other methods do and therefore may be readily used by engineers. The problem of seepage from (into) an unconfined semiinfinite aquifer into (from) an adjacent reservoir that has a sudden change of water level is revised. Comparisons with the Polubarinova-Kochina series expansion are done. Superelevation of the water table in a wetting regime compared to a drainage regime is quantified by the value of sorptivity (desorptivity). The absolute values of sorptivity and desorptivity diverge as the amplitude of the reservoir level change increases. A problem of a steady flow into (from) a constant head well from (into) an unconfined leaky aquifer is also examined. The water table elevation, well rate, and volume of the cone of depression (injection) are calculated.

Procedures were developed and tested for quantifying seepage losses in unlined irrigation canals for reaches on the order of 100 ft (30 m). The procedure is appropriate for trapezoidal canal sections underlain by clay with a layer of more permeable material at depth. These conditions prevail throughout much of the Central Nebraska Public Power and Irrigation District (Central) canal system. The procedure uses electrical resistivity (ER) measurements while canals are in service to determine the resistivity of the underlying clay layer. ER data were correlated to canal depth and then to seepage rate. Seepage rates were determined using seepage meters. Field test sites showed seepage rates from 0.07 to 0.62 (ft³⋅s) (ft width⋅mi) [0.0041 to 0.036 (m³⋅s) (m width⋅km)]. Accuracy is approximately ±20%, comparable to that achieved using stream-gauge methods, which can achieve a ±5% error of total canal discharge. The ER approach, however, can easily pinpoint seepage zones more precisely than accomplished using stream gauging and will allow Central to reduce the length of canal lining projects. The methods outlined in this paper are simple, relatively quick, and can provide seepage estimates for short reaches in extensive conveyance systems. Such information will be valuable to managers as they make decisions regarding water conservation and ground-water management and where to invest limited resources.

Realizing the need for improvements in the operation and management of the main canal system through performance assessment, the MIKE 11 hydraulic model was applied to the Right Bank Main Canal system of the Kangsabati project, West Bengal, India. The canal system has a length of 137 km with a delivery area of about 88,867 ha. The model was calibrated for a 15-day irrigation period during 1997 and validated for one irrigation period each in 1995, 1996, and 1997. The calibrated parameters, which are the global resistance number and gate contraction coefficient, are found to be 50 and 0.63, respectively, whereas local resistance numbers range between 15 and 60. Daily observed and simulated flows are in close agreement for the head and middle reaches during calibration and validation periods. However, at the tail reach some mismatch is found. These results are also supported by the statistical tests. Further, a performance ratio (a ratio of the observed flow rate to the scheduled flow rate) is used as an indicator for assessing the degree of uniformity in flow deliveries along the length of the canal. A sharp decline is seen in the performance ratio along the length of the canal because most of the distributaries of the head and middle reaches have drawn more than their desired shares. The tail reach is more sensitive to discharge deviation at the system source than at the head and middle reaches. The results of the study show that MIKE 11 can be successfully used for a large and complex irrigation system (Right Bank Main Canal) to evaluate its performance in the absence of observed flow data and assist in developing comprehensive irrigation management plans for a cropping season.

To avoid sediment deposits in the downstream portion of irrigation channels, the outlets must extract a concentration equal to or higher than the one in the channel. In Pakistan, although channels have been designed in that way, the present management leads to a general deposition of sediments. A few concentration measurements and the use of a 3D model of one outlet suggest that the sediment discharge in the outlet is close to the one contained in the current tube entering the outlet. Finally, a simple model with one parameter is proposed to compute the sediment concentration in the outlet.

This paper presents an experimental investigation on noncohesive sediment pickup under a unidirectional steady-uniform stream flow on streamwise steeply sloping (down slope and adverse) sedimentary beds. The characteristic parameters affecting the sediment pickup, identified based on the physical reasoning and dimensional analysis of the sediment particle movement under stream flow, are the transport-stage parameter, particle parameter, and geometric standard deviation of sediment particles. A large number of experiments (426 runs) were carried out in two long rectangular ducts (closed-conduit flow) with nine types of sediments (six uniform and three nonuniform sediments), having a variation of bed slope from −15° (adverse slope) to 25° (down slope). In an open channel flow (laboratory flume study), the 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. Measurements included flow discharge and sediment pickup rate. The bed shear stress for a particular run was computed considering side wall correction. The experimental data were used to determine the equation of sediment pickup function through a regression analysis. The equation is adequate to estimate sediment pickup not only on horizontal and mild slopes but also on steep and adverse slopes.

The model most widely used to represent sprinkler irrigation distribution parameters is based on numerical solutions to the normal cumulative probability density function. For most practical irrigation design and management applications, numerical solutions are too laborious. One other study reported analytical approximations for several irrigation distribution parameters derived from the normal model. The estimation error resulting from those approximations were variable over the operational range of irrigation uniformity and irrigation adequacy and were quite high in some ranges. In this note, more accurate analytical approximations are presented for the distribution coefficient, the application efficiency, the water requirement efficiency, the deficiently irrigated volume, and the average deficit over the deficiently irrigated area. On average, over the entire operational range of irrigation uniformity and irrigation adequacy, the new approximations are about an order of magnitude more accurate than the previous approximations and introduce negligible error for most practical applications.

Many irrigated areas in the United States and abroad overlie unconfined aquifers. The soils are coarse textured with low water-holding capacity, and irrigation water is frequently necessary for crop growth. Many of these areas experience elevated nitrate levels in water from shallow domestic wells. It is often observed that the nitrate plumes are stratified with depth with the highest concentration just near the surface. The irrigation wells are generally screened toward the bottom part of the aquifer where the material is coarse and the available drawdown is greater. Pumpage is typically cyclic but can be somewhat continuous during drought conditions. It is often believed that the floating nitrate plume is recycled by the irrigation wells. Simulations were carried out to show that slight deepening of domestic wells in both stratified and more homogeneous aquifers could eliminate nitrate problems for many domestic wells even in close proximity of irrigation wells.

A study was conducted to revise the procedure for adjusting discharge measurements in a cutthroat flume subjected to settlement in free-flowing conditions. A series of 57 tests was performed on six different size cutthroat flumes. Each flume was systematically adjusted to simulate settlement in both the lateral and the longitudinal directions. Discharge inaccuracies were then recorded for each settlement condition. The test results were combined with the results of three cutthroat flumes previously reported. The coefficients describing the flume geometry, C_dlat and C_dlong, were determined to be significantly different than previously indicated. New expressions for C_dlat and C_dlong were formulated for inclusion into the previously reported discharge measurement adjustment procedure.