Journal of Irrigation and Drainage Engineering

Detailed measurements were made of the levels and distributions of salts present in representative soil profiles and fields and associated tailwaters in the Imperial Valley of California. The findings showed that the potential salinity-pickup hazard may be greater in this valley that is dominated by cracking soils than classical theory would predict. Salts that would otherwise be “isolated” in seedbeds or leached downward during irrigations are more “exposed to” and “picked up by” the runoff water than previously recognized as a result of the flow of the irrigation water throughout the beds and horizontally in the topsoil via the extensive network of cracks and fractures that form in the cracking soils. As a result, the pattern of salinity within the beds of such soils is one-dimensional, rather than the expected, classical two-dimensional pattern. Salt content in the tailwater associated with cracking soils was higher and sustained over longer periods of time than in the case of noncracking soils.

Cracking clay soils pose unique and complex water and salt management problems. Crack volumes greatly affect water and salt movement in these soils. The purpose of our study was to determine the effect of crack flow on tail water quality and the contribution of crack flow to salt leaching. Our experiment was conducted on two farmers fields near Brawley, California. One field contained a nonswelling sandy loam soil and the other contained a high shrink-swell clay soil. Both fields were planted to alfalfa on shallow beds. Soil samples were taken prior to bromide treatment, after treatment, and after several subsequent irrigations. Three experiments were conducted on each soil: (1) initial bromide spike injection; (2) midstream bromide spike injection; and (3) infiltration box bromide injection. Surface water was sampled during bromide application and during subsequent irrigations. On the noncracking soil the water and salt moved to a depth of 60–75 cm. During subsequent irrigations salt was leached to deeper depths and surface waters had only small amounts of bromide. In the clay soil, water and salt rapidly filled and flowed in the soil cracks. Significant quantities of bromide were found at the 90–120 cm depths. Surface waters were laden with bromide during subsequent irrigations, and bromide was transported down the furrow by irrigation water. Traditional water and salt transport concepts for porous media are not applicable to cracking clay soils.

The interactive effect of irrigation methods and spatial variability of saturated hydraulic conductivity (K_s) on solute transport was determined with the combined use of a two-dimensional deterministic solute transport model and a stochastic parameter generator. In a homogeneous K_s field, the time required to infiltrate a prescribed amount of water or chemical increased from sprinkler to furrow to drip irrigation. Furrow irrigation appeared to leach the chemical more rapidly than either drip or sprinkler irrigation. Assuming the spatial distribution of K_s to be a stationary stochastic process, increased spatial variability in K_s reduced the infiltration rate. When K_s is spatially correlated, sprinkler irrigation appeared to be less susceptible to cause ground-water contamination than furrow or drip irrigation. The concentration distributions in the uncorrelated K_s field were not very different from those in the homogeneous field.

A coupled model was used to simulate the hydrology of a 22 km² agricultural marsh characterized by a very dense channel network composed of 1,800 storage basins closely connected with 500 km of ditches. The model couples the field-scale drainage model SIDRA with the hydraulic model MAGE. The SIDRA-MAGE model simulates the hydrology of independent fields, either subsurface drained or undrained, and routes outflows from the fields through numerous basins, ditches, and canals to the outlet of the marsh. Evapotranspiration from surface water bodies colonized by aquatic plants, seepage from ditches, and management of water gates in the channel network are considered. Calibration and validation of the SIDRA, MAGE, and coupled SIDRA-MAGE models are not presented in the paper. The study focuses instead on the simulation of the hydrological effects of eight subsurface drainage schemes for a 72-h event. As the percentage of marsh drained increases from 0 to 88%, peak flow, mean flow, and total outflow volume at the marsh outlet are increased by 7, 27, and 27%, respectively. However, there is no significant changes in outflows at the marsh outlet when the percentage of marsh drained increases from 44 to 62%, even though flow from fields was predicted to increase by 8%. This is attributed, first, to a small change of the channel network efficiency, which is less than 8%, and, second, to the distribution of drained areas within the marsh. Drainage effects on the hydrology of the marsh were also simulated for summer conditions. Reduction of the channel network length and the area of basins by 72 and 84%, respectively, would reduce evapotranspiration losses from surface water bodies by 30% and save up to 11,000 m³/day of freshwater.

Sideweirs for combined sewers in converging U-shaped channels are analyzed. The results are applicable for storm sewers. Based on extended model observations and a hydraulic approach, the characteristics of sideweir flow are determined. These include the surface profile, the discharge distribution, the end depth ratio, and the energy losses due to lateral outflow. All results are presented in terms of approach conditions, and it is demonstrated that the approach Froude number is the significant flow parameter. For sideweirs with a relatively small weir height, the flow may become unstable and set up a surging phenomenon in the approach pipe. After careful experimentation, the instability was effectively removed with a so-called end plate. This device is positioned at the end wall of the sideweir and is able to stabilize the vorticity interface between the bottom and the surface vortices. The end plate can simply be added to existing designs.

An approach for draining water from unsaturated soil is presented that utilizes a sloping system of a fine sand overlying a coarser sand or gravel. In this configuration, referred to as an unsaturated drainage layer, the sand laterally drains water that accumulates above the capillary break provided by the coarse material. Drainage capacity is maximized when the sand layer has sufficient moisture to be relatively conductive, yet remains unsaturated so as to prevent failure of the capillary break. Results from field tests indicate substantial downward infiltrating water can be laterally diverted with an unsaturated drainage layer. Numerical simulations are presented that illustrate the potential effectiveness of unsaturated drainage layers to divert sufficient infiltrating water and prevent the development of positive pore water pressures.

High contents of soluble salts accumulated in a soil can significantly decrease the value and productivity of agricultural lands. Present recommendations for reclamation are usually based only on relatively simple and often empirical relations. In this paper we give a brief overview of many unique physical and chemical factors important to reclamation contained in the one-dimensional multicomponent transport model UNSATCHEM and demonstrate its use for evaluating the reclamation of a sodic soil using various amendments and management strategies. We evaluate particular reclamation strategies based on the following considerations: (1) the quantity of water needed; (2) the quantity of amendments to be used; and (3) the time required for reclamation to be completed. Our study concentrates primarily on such chemical reactions as cation exchange, precipitation, and dissolution of solid phases (reclamation amendments) and on the effect of solution composition on the soil hydraulic properties and corresponding changes in the water flow and solute transport rates.

A seasonal furrow irrigation model was constructed from soil moisture, kinematic-wave hydraulic, and economic optimization models to study the effects of heterogeneity in water balance, soil water holding properties and rooting depth, and infiltration functions on furrow irrigation design (flow rate and cutoff time), bean yield, and net return to water. Irrigation designs achieving nearly 100% irrigation adequacies were unchanged by heterogeneity in water balance, soil water properties, and rooting depth, but both bean yield and net return to water decreased with increasing heterogeneity. Irrigation designs were sensitive to infiltration characteristics and to irrigation interval. At a given irrigation interval, bean yield was insensitive to infiltration characteristics but consistently decreased with increasing irrigation interval. Net return to water was less for spatially variable infiltration functions as compared to homogeneous infiltration conditions. Using mean evapotranspiration (ET) of a grass reference crop (ETo) resulted in slightly higher bean yield and net return to water as compared to observed ETo (1992 season values of grass reference crop ET). Spatial and temporal variability in infiltration gave the same optimal irrigation interval (10 d) under both ETo conditions for the chosen irrigation criteria (80% irrigation adequacy at cutoff time). Both ETo conditions led to essentially the same design inflow rates. Errors in inflow volume were less than 12 and 3.5%, respectively, for individual irrigations and on a seasonal basis. Irrigation scheduling and surface irrigation design can be forecast at the beginning of the growing season for the conditions studied using historical mean ETo if spatial and temporal variability of infiltration are considered.

Distributed, physically based watershed and irrigation advance models require robust infiltration estimation capabilities. The empirical Green and Ampt (GA) equation of infiltration is a popular method for estimating infiltration. The GA parameters have physical basis and considerable prior research has focused on relating these parameters to soil textural classification. However, the original GA method is limited in that it is applicable only for a single ponding period. An explicit Green and Ampt redistribution (GAR) technique is developed herein to estimate interstorm redistribution of soil water and allow multiple ponding simulations using the GA methodology. Soil water redistribution during interponding periods is estimated using the physical constructs of water retention curves without hysteresis. A GA wetting-front suction parameter scaling function is derived for use between arbitrary soil water contents. Simulations on 11 soil textural classifications comparing the GAR method with the numerical solution of Richards equation reveal that the GAR method is a viable technique for estimating infiltration for multiple pondings with acceptable error.

The Food and Agriculture Organization of the United Nations (FAO) version of the Penman-Monteith method (FAO-PM) has recently been proposed as the standard for estimating reference evapotranspiration (ETo). A standard, well-watered reference environment for the data used for the ETo estimate has also been proposed. At those sites providing a reference environment the average minimum air temperature (T_n) is approximately the same as average dew point temperature (T_d). For this reason and because the computation of T_d is plagued with errors, the FAO-PM can be modified to use T_d=T_n. This modification is referred to as FAO-MOD. However, most weather stations lack the intensive input required by the FAO-PM. Few weather stations provide a standard environment. Errors in measuring the required weather variables are common. Under these conditions, equations with few weather parameters are recommended. The Hargreaves et al. (1985) method (ETH) is one of the simplest and most accurate empirical equations. The ETH method and ETH corrected for site aridity (ETH-MOD) are compared with the FAO-PM and FAO-MOD. Comparisons are made for many different locations and conditions. The results indicate that the differences in ETo values computed by the different methods are minor when compared with the uncertainties in estimating actual crop evapotranspiration.

Analysis of diversion of flow has received considerable importance in existing literature. In this study, diversion of flow through a lateral slot is analyzed using the spatially varied flow equation and the elementary discharge coefficient function. The implications of treating flow through a side slot as through a small orifice, as reported in the literature, are also studied. For different combinations of slot openings and sill heights, data for a set of 110 runs have been obtained. Using these experimental observations, equations for the elementary discharge coefficient are obtained for small as well as large orifice formulations of flow through a lateral slot. The elementary discharge coefficient is mainly dependent on slot geometry. Using the developed expressions of the elementary discharge coefficient, it is shown that more than 90% of the data are represented within an error band of ±10%.