Journal of Irrigation and Drainage Engineering

The shortage of reservoir water is the main constraint in establishing stable water management programs in the Muda Irrigation Project, Malaysia. In this study, a water balance equation was derived to evaluate the performance of the project. The water balance components were modeled without calibration, and compared with measured data, whenever possible. The overall project efficiencies for the main and off seasons were also obtained. A reservoir simulation model was developed from which simulated storage capacities compared satisfactorily with the observed storage capacities. An optimization model was also developed to solve the water resources management of the project in a computationally satisfactory manner. The optimal reservoir storage, optimal irrigation demand, and optimal reservoir release (i.e., optimal reservoir operating policy) were computed. The mean (1987–1997) optimized total water requirements for the dry and wet seasons were also computed as were the optimal contributions by rainfall, reservoir, uncontrolled river flow, and recycled water. The optimized water requirements were compared with other authors’ computed water requirements and found to be satisfactory. The mean water balance components results for different months were stored in Geographic Information System (GIS) databases, analyzed, and displayed as the monthly crop water requirements maps. The integration of the water balance model together with the models for reservoir simulation, efficiency, optimization, and GIS holds much promise in the analysis of optimal allocation of water resources of a project.

This paper shows that the initial value of any instantaneous unit hydrograph (IUH) should be a positive value, however small. This unique characteristic is clearly demonstrated by the IUH of overland flow derived for a plane of uniform width based on the kinematic wave solution. Not only is a nonzero initial value a distinct and required characteristic of the IUH, it also provides an additional condition for determining the parameters of the IUH. The gamma probability density function has gained wide acceptance as an IUH for a watershed that is represented by a series of identical linear reservoirs. It has an initial zero value. This study shows that the model used in deriving the popular gamma probability density function IUH, with its characteristic zero initial value, is conceptually unsound. This paper presents a true form of IUH which results from a watershed represented by a series of identical linear reservoirs. Unlike the conceptual model used in deriving this gamma probability density function IUH, in this model the unit effective rainfall is distributed uniformly across all of the reservoirs. The resulting IUH contains both a gamma function and an incomplete gamma function, and it possesses the required positive initial value. This IUH may be viewed as a counterpart of the IUH derived from the kinematic wave solution and is named the pseudokinematic-wave IUH (PKW-IUH). By extending the PKW-IUH and using different weights for the series of linear reservoirs to reflect the shape of the watershed, a general form of IUH can be derived. For practical applications a three-parameter IUH, with a storage coefficient and two weights for the linear reservoirs, is proposed. Finally, an example to demonstrate the application of the proposed three-parameter IUH in predicting direct runoff is presented and the results are compared with the runoff predicted by the universally accepted gamma probability density function IUH.

A power-law channel is a generalized form of a channel and includes parabolic and triangular cross sections. For an exponent $m<0.5$ in the power law, the relative wetted perimeter has been estimated from a series expansion truncated to four terms. For values of the exponent $m⩾0.5$ the relative wetted perimeter has been estimated using an appropriate non-linear interpolation expression. A table to estimate relative wetted perimeter based on these expressions is presented for design purposes. With these expressions for relative wetted perimeter, and using the Lagrange method of undetermined multipliers, for any given maximum side slope, the area and/or wetted perimeter is minimized subject to the equality constraint of a uniform flow (Mannings) equation. Using this technique, for any given side slope, the exponent of the power-law channel can be determined and hydraulically efficient power-law channels can be designed. Optimized power-law channels are compared with trapezoidal and parabolic channels. The existing parabolic design of the Pehur High Level Canal, Pakistan is compared with an optimum power-law channel.

The present study investigated the relationship between the crop water stress index (CWSI) and soil moisture for surface irrigated cotton (Gossypium hirsutum, Delta Pine 90b) at Maricopa, Arizona during the 1998 season. The CWSI was linked to soil moisture through the water stress coefficient $K_s$ that accounts for reduced crop evapotranspiration when there is a shortage of soil water. A stress recovery coefficient $K_{\mathrm{rec}}$ was introduced to account for reduced crop evapotranspiration as the crop recovered from water stress after irrigation events. A soil water stress index (SWSI) was derived in terms of $K_s$ and $K_{\mathrm{rec}}\text{.}$ The SWSI compared reasonably well to the CWSI, but atmospheric stability correction for the CWSI did not improve comparisons. When the CWSI was substituted into the SWSI formulation, it gave good prediction of soil moisture depletion (fDEP; when to irrigate) and depth of root zone depletion $(D_r;$ how much to irrigate). Disagreement was greatest for $\mathrm{fDEP}<0.6$ because cotton is less sensitive to water stress in this range.

A remote sensing package called the agricultural irrigation imaging system (AgIIS) aboard a linear move irrigation system was developed to simultaneously monitor water status, nitrogen status, and canopy density at one-meter spatial resolution. The present study investigated the relationship between water status detected by AgIIS and soil moisture for the 1999 cotton (Gossypium hirsutum, Delta Pine 90b) season in Maricopa, Ariz. Water status was quantified by the water deficit index (WDI), an expansion of the crop water stress index where the influence of soil temperature is accounted for through a linear mixing model of soil and vegetation temperature. The WDI was best correlated to soil moisture through the FAO 56 water stress coefficient $K_s$ model; stability correction of aerodynamic resistance did not improve correlation. The AgIIS did provide field images of the WDI that might aid irrigation scheduling and increase water use efficiency.

The fate of nitrogen in the soil is of major concern because of the potential hazard for nitrogen, applied in excess of the natural decomposing capacity of the soil, to contaminate shallow and deep aquifers. For the prediction of the nitrogen behavior in soils simulation models are frequently used. In this study the transport and fate of nitrate within the soil profile was analyzed by comparing historic field data with the simulation results of two mathematical models, i.e., the water and agrochemicals in the soil crop and Vadose environment (WAVE) and DRAINMOD-N. After calibration and validation of both models, they were used to simulate the nitrogen transport and transformation of the Hooibeekhoeve experiment, situated in the sandy region of the Kempen, Belgium, for a 30-year (1969–1998) period. In the analysis a continuous cropping with maize was assumed. Comparison between experimental measured and simulated state variables indicate that the nitrate concentrations in the soil and nitrate leaching to drains are controlled by the fertilizer practice, the initial conditions and the rainfall depth, and distribution. Furthermore, the study reveals that the models used give a fair description of the nitrogen dynamics in the profile root zone at field scale. It was concluded that the calibrated models are useful tools to optimize the nitrogen application strategy resulting in an acceptable level of nitrate leaching for a long period as a function of the combination “climate–crop–soil–bottom boundary condition.”

A brief history of development of the 1985 Hargreaves equation and its comparison to evapotranspiration (ET) predicted by the Food and Agricultural Organization of the United Nations (FAO) Penman-Monteith method are described to provide background and information helpful in selecting an appropriate reference ET equation under various data situations. Early efforts in irrigation water requirement computations in California and other arid and semiarid regions required the development of simplified ET equations for use with limited weather data. Several initial efforts were directed towards improving the usefulness of pan evaporation for estimating irrigation water requirements. Similarity with climates of other countries allowed developments in California to be extended overseas. Criticism of empirical methods by H. L. Penman and others encouraged the search for a robust and practical method that was based on readily available climatic data for computing potential evapotranspiration or reference crop evapotranspiration $\left({\mathrm{ET}}_o\right)\text{.}$ One of these efforts ultimately culminated in the 1985 Hargreaves ${\mathrm{ET}}_o$ method. The 1985 Hargreaves ${\mathrm{ET}}_o$ method requires only measured temperature data, is simple, and appears to be less impacted than Penman-type methods when data are collected from arid or semiarid, nonirrigated sites. For irrigated sites, the Hargreaves 1985 ${\mathrm{ET}}_o$ method produces values for periods of five or more days that compare favorably with those of the FAO Penman-Monteith and California Irrigation Management Information Services (CIMIS) Penman methods. The Hargreaves ${\mathrm{ET}}_o$ predicted 0.97 of lysimeter measured ${\mathrm{ET}}_o$ at Kimberly, Idaho after adjustment of lysimeter data for differences in surface conductance from the FAO Penman-Monteith definition. Monthly ${\mathrm{ET}}_o$ by the 1985 Hargreaves equation compares closely with ${\mathrm{ET}}_o$ calculated using a simplified, “reduced-set” Penman-Monteith that requires air temperature data only.

The temporal and spatial distribution of salinity in the field is usually expressed by simple averaging of the data. The high values of standard deviation that usually exceed 50% of the average salinity make it impossible to reach valid conclusions. A method that normalizes soil salinity data was developed and its advantages are presented.