Journal of Irrigation and Drainage Engineering

Evapotranspiration (ET) is a major component of the hydrologic cycle and its accurate estimation is essential for hydrological studies. In the past, various estimation methods have been developed for different climatological data, and the accuracy of these methods varies with climatic conditions. Therefore, a user-friendly decision support system (DSS) was developed for estimation of the reference value, ETo. For given data availability and climatic conditions, the developed model estimates ETo with the best available method out of the nine methods. The ETo estimation methods are based on combination theory, radiation, temperature, and pan evaporation; the model selects the best ETo estimation method based on ASCE rankings. In order to evaluate the DSS, various tests were conducted with different data availability conditions for five climatological stations (Davis, Bellary, Kharagpur, Jagdalpur, and Bombay). The decisions made by the model exactly matched the ASCE rankings. For the three climatic stations Davis, Jagdalpur, and Kharagpur, ETo values were estimated by all applicable methods using the user defined options menu and were compared with the Penman-Monteith ETo estimates, which were taken as the standard. Based on the weighted average standard error of the estimate, the Hargreaves and FAO-24 Blaney-Criddle methods ranked first, respectively, for the Davis and Jagdalpur stations. The 1982 Kimberly-Penman method ranked first for Kharagpur. The DSS model is a user-friendly tool for estimating ETo under different data availability and climatic conditions.

Development of water saving measures requires a thorough understanding of the water balance. Irrigation performance and water accounting are useful tools to assess water use and related productivity. Remote sensing and a hydrological model were applied to an irrigation project in western Turkey to estimate the water balance to support water use and productivity analyses. Remote sensing techniques can produce high spatial coverage of important terms in the water balance for large areas, but at the cost of a rather sparse temporal resolution. Hydrological models can produce all the terms of the water balance at a high temporal, but low spatial resolution. Actual evapotranspiration for an irrigated area in western Turkey was calculated using the surface energy balance algorithm for land (SEBAL) remote sensing land algorithm for two Landsat images. The hydrological model soil-water-atmosphere-plant (SWAP) was setup to simulate the water balance for the same area, assuming a certain distribution in soil properties, planting dates and irrigation practices. A comparison between evapotranspiration determined from SEBAL and from SWAP was made and differences were minimized by adapting the distribution in planting date and irrigation practice. The optimized input data for SWAP were used to simulate all terms of the accumulated water balance for the entire irrigation project, and subsequently used to derive the irrigation performance indicators. The innovative methodology presented is attractive as it diminishes the need of field data and combines the strong points of remotely sensed techniques and hydrological models.

To evaluate the performance of irrigation systems, different indicators have been used by researchers. In this study some of the water management problems of three different irrigation systems in Iran are presented. In addition, the water delivery performance of the Doroodzan Irrigation and Drainage Network in southern Iran is evaluated in detail. The analyses included wet and dry seasons and were based on the indicators of overall project water delivery efficiency $\left(e_p\right)$ and the monthly water requirement of crops. The distribution and conveyance of water in the Doroodzan Irrigation Network was unreliable in both seasons. With an overall project efficiency of around 46%, about 20% of the total delivered water was distributed unreliably. Water distribution equity along tertiaries was also found to be poor. A contributing factor could be the poor operation and maintenance of gates.

In this paper, the contribution of different sources of variability to irrigation water depth variability was quantified using a combination of variance techniques. This method was applied using field measurements from irrigation events performed on a loamy soil with a low-infiltration rate. Infiltration variability was estimated with blocked furrow infiltrometers. The assumptions made for the application of the combination of variance techniques proved to be valid. The major variability source turned out to be the soil intake characteristics, whose variance accounted for 45–71% of the variance in infiltrated depth under first irrigation conditions. Opportunity time and wetted perimeter were less variable in subsequent irrigations and the soil intake characteristics variability accounted for a percentage of total variance beyond 76%, being at times beyond 95%. The combination of variance techniques can be used to complement standard evaluation methods in order to take into account the influence of different variability sources.

Previous continuous-uniform outlet discharge approaches for the hydraulic analysis of irrigation laterals are generally valid for large (theoretically) infinite number of outlets. For a finite number of outlets, however, these approaches may lead to errors in hydraulic computation. A new continuous-uniform outflow approach that takes into account the effect of the number of outlets on the lateral hydraulics is presented. A new analytical equation describing the energy line shape along uniform sprinkle and trickle irrigation laterals and manifolds is developed. The effect of ground slope and velocity head on hydraulic computation is also considered. The method is however restricted by the simplified assumption of equal outlet discharge. An alternate improved analytical method considering the effect of non-uniform outflow distribution along the lateral is also included. Analytical expressions for determining the inlet pressure head and global statistical parameters characterizing the outflow distribution (Christiansen uniformity coefficient, pressure head variation) are developed for design and evaluation purposes. Comparison tests with an accurate numerical stepwise method indicated that the proposed simplified approach is more accurate than other previous works particularly when the number of outlets is relatively small. The improved method is the most accurate method for all cases examined even for low levels of uniformity.

A pressure reducing pipe (PRP), used at the inlet of a lateral in microirrigation systems to improve water application uniformity, is developed. It is composed of an internal spiral and a sleeve pipe and is made of molded polyethylene plastic. It functions to dissipate a pressure head that exceeds the required pressure head of a lateral inlet by directing water flow through the spiral grooved path and creating a certain head loss. An improved design method for submain units using the pressure reducing pipes is based on the hydraulics of the submain unit and the characteristics of the head loss in the PRP. The effects of the PRP on water application uniformity in a submain unit are analyzed by computer simulation. The submain unit with the PRP has higher uniformity than that of the commonly used method. After choosing the PRP, the permissible range of pressure variation in a submain will not need to be considered for the submain length in a layout.

When numerical models are used for estimating the water flow and solute transport at field scale, modelers are confronted with the issue of spatial variability of soil hydraulic properties. In this paper, the concept of effective soil hydraulic parameters is used to study water and solute transport to tile drains using a three-dimensional deterministic model. A direct averaging approach is used to interpret the field as a single equivalent soil. The results obtained using the direct averaging approach and heterogeneous soils were compared. It is concluded that effective soil hydraulic properties can be used for heterogeneous soils in numerical simulation models to simplify calculations.

An efficient method that uses discrete ramp kernel is proposed for obtaining the piezometric head in an aquifer due to an arbitrary variation in stream stage considering stream resistance. The method assumes straight line variation between two consecutive points in representing the arbitrary stream stage variation. Expression for the ramp kernel is derived for homogeneous and isotropic aquifer conditions. Using the method, the stream resistance and hydraulic diffusivity of the aquifer are estimated for a set of published data. It is observed that the hydraulic diffusivity should be estimated along with the stream resistance for a better estimation of aquifer diffusivity.