Journal of Irrigation and Drainage Engineering

Modeling of crop yield and root zone hydrosalinity usually requires a large number of parameters that are often expensive to obtain and have associated measurement errors. Consequently, identifying the most relevant parameters, and their contributions to the uncertainty of the output, might be utilized as a basis to focus research resources efficiently. Global sensitivity analysis (GSA) is a powerful tool that can be utilized to achieve this goal. However, some GSA methods perform better than others, which introduces the risk of mistakenly prioritizing a secondary parameter while neglecting a primary one. This paper evaluates four GSA methods utilized to rank the importance of a total of 18 input parameters with respect to five performance indices using a variable saturated flow and transport model. Some of the major findings of this research are (1) the crop-yield variance is controlled largely by only four of the 18 parameters that were considered in this study; (2) the van Genuchten pore-size parameter was found to be very important to the relative crop-yield prediction and the water-availability index; and (3) the variance-decomposing method, the screening method, and the Monte Carlo–filtering method are generally consistent in their performance, whereas the partial-correlation coefficient method is significantly different.

A root uptake model developed and validated for a uniform crop root zone in a semiarid agroclimate is tested for its efficacy in predicting moisture depletion in a multilayer crop root zone in a subtemperate, subhumid agroclimate. Evaluation of moisture uptake prediction efficiency of model in uniform, vis-à-vis multilayer crop root zone, done on the basis of secondary experimental data of three crops, indicates significant improvement in predicted moisture uptake when a multilayered crop root zone is considered. To validate the enhanced prediction efficiency of the model, in the case of a multilayer crop root zone, field experiments on two Indian crops, i.e., Indian mustard (Brassica Juncea) and Wheat (Triticum-aestivum), are conducted in the subtemperate, subhumid agroclimate of Solan, Himachal Pradesh, India. Model performance indicators exhibit good agreement between model-predicted soil-moisture parameters and experimental results. The results demonstrate the utility of the root water uptake model across semiarid, subtemperate, subhumid agroclimates and establish its efficacy for nonuniform soils having variations in soil moisture properties along depth in predicting soil moisture in the crop root zone.

Alternate-furrow fertigation has shown potential to improve water and fertilizer application efficiency in irrigated areas. A combination of simulation and optimization approaches permits researchers to identify optimum design and management practices in furrow fertigation, resulting in optimum cost, irrigation performance, or environmental impact. The objective of this paper is to apply one-dimensional (1D) surface and two-dimensional (2D) subsurface simulation-optimization models to the minimization of nitrate losses in two types of alternate-furrow fertigation, as follows: (1) variable alternate-furrow irrigation, and (2) fixed alternate-furrow irrigation. For comparison purposes, optimizations are also reported for conventional furrow irrigation. The model uses numerical surface fertigation and soil-water models to simulate water flow and nitrate transport in the soil surface and subsurface, respectively. A genetic algorithm is used to solve the optimization problem. Four decision variables (inflow discharge, cutoff time, start time, and duration of fertilizer solution injection) were optimized to minimize the selected objective function (nitrate loss) for two fertigation events performed during a maize-growing season. The simulation-optimization model succeeded in substantially reducing the value of the objective function as compared with the field conditions for all irrigation treatments. In the experimental conditions, optimization led to decreased inflow discharge and fertilizer injection during the first half of the irrigation event. This was because of the high potential of the field experiment to lose water and nitrate through runoff. In the optimum conditions, alternate-furrow fertigation strongly reduced water and nitrate losses compared with conventional furrow irrigation. The simulation-optimization model is a valuable tool for alleviation of the environmental impact of furrow irrigation.

Agricultural production is a significant contributor to the economy of Jamaica, which is situated in the northwestern Caribbean Sea; this production is heavily dependent on seasonal rainfall because only approximately 10% of Jamaica’s cultivated lands are irrigated. Drought is a disastrous natural phenomenon that has a significant impact on socioeconomics, agriculture, and the environment. In the 2000–2001 drought experienced in Jamaica, there were crop losses amounting up to US$6 million. Hence, drought index information is essential for better planning for drought impacts and allows for the introduction of mitigation measures in the agricultural sector. Therefore, the objective of this paper is to evaluate the suitability of both the Standardized Precipitation Index (SPI) and the Normalized Difference Vegetation Index (NDVI) in reflecting water stressed conditions and irrigation demand requirements for three agricultural sites: Savanna-la-Mar in the parish of Westmoreland, Beckford Kraal in the parish of Clarendon, and Serge Island in the parish of St. Thomas, all in Jamaica. These sites were selected based on soil characteristics, historical rainfall data, and farming practices. The results indicate that the NDVI provides a suitable representation of these areas for only the driest months of the year, and that either the one-month or three-month SPI was found to be more representative of soil moisture conditions. Furthermore, a correlation analysis was also conducted between the SPI and soil moisture for El Niño years only, because the El Niño/Southern Oscillation phenomenon has been responsible for many of the droughts Jamaica has experienced. In these years, good correlations between soil moisture and the one-month and three-month SPI were obtained in some wet months, in addition to the dry months. This paper provides soil moisture values for all of the different categories and values of the SPI relating to water scarcity. It also provides irrigation requirements for the “moderately dry” and “severely dry” SPI drought categories.

Evapotranspiration (ET) is an important component of the hydrologic cycle, especially for irrigated agriculture. Direct methods of estimating reference ET are difficult or require many weather variables that are not always available at all weather stations. The Hargreaves equation (HG) requires only measured daily air temperature data and computed extraterrestrial radiation for ET estimates. Unless it is regionally calibrated, however, HG often tends to systematically overestimate or underestimate ET. This equation was evaluated under semiarid conditions in the Bekaa Valley of Lebanon using 16 years of complete daily climatic data from the Terbol weather station. The HG results were compared to ET estimates obtained from the FAO56 Penman Monteith equation (PM), which was used as a standard. The original HG equation overestimated ET by 23, 17, and 12% for daily, weekly, and monthly ET, respectively, as compared to PM. The results of a simple linear regression applied to obtain the calibrated HG coefficients for all three time steps showed that the calibrated equation improved the accuracy of the estimation to 3, 2, and 1% difference from ET computed by the PM method, with root mean square error (RMSE) of 0.48, 0.33, and $0.25\mathrm{mm}{\mathrm{d}}^{-1}$ for daily, weekly, and monthly ET, respectively. Additional improvement in HG estimation accuracy was achieved by adding the wind speed using a backward variable selection method. This method resulted in only a slight improvement, reaching less than a 1% difference for all timescales and RMSE of 0.46, 0.31, and $0.22\mathrm{mm}{\mathrm{d}}^{-1}$ for daily, weekly, and monthly ET, respectively. Thus, when only temperature data are available, the calibrated HG equation is recommended for use in the semiarid conditions of Lebanon, and when complete and reliable weather data exist, the use of the standard FAO56 PM equation is recommended.

The permeability of uniform and mixed-size (volume based) tire chips under various overburden pressures is determined. Tire chips sizes of 5.08, 10.16, and 15.24 cm are used. The permeability tests are conducted using sample layers of 15.24-, 30.48-, and 45.72-cm thickness. The vertical and lateral permeability data of uniformly sized chips under no overburden are in the range of coarse gravel. The mixed-size tire chips have permeability in the range of coarse to very coarse gravel under no overburden. Under overburden, the compressibility of the uniformly sized tire chips increases with tire-chip size and decreases with layer thickness. There is a general trend of decrease in permeability with increase in overburden pressure. In the case of uniformly sized chips under overburden, the 5.08-cm chips have the highest permeability and the 15.24-cm chips have the lowest permeability. The mixed-size tire chips have higher permeability than the uniformly sized tire chips. In general, the permeability of uniform and mixed-size tire chips is in the range of coarse gravel.

This study focuses on the discharge coefficient ($C_d$) for flow into orifices cut with a circular bit perpendicular to and along the centerline of a round pipe, a common configuration on perforated risers installed as the principal outlets for stormwater detention or sediment basins. When predicting flow, the orifice area is generally defined as the area of the bit used to cut the hole, but the true orifice area is larger than the bit due to the curvature of the riser pipe. Four different descriptions for orifice area were tested while attempting to fit $C_d$ to measured discharge experiments. The tests showed that $C_d$ decreases as the orifice diameter approaches the diameter of the riser. Photographic imagery shows that the reduced $C_d$ is due to lateral flow from the riser’s curved sides decreasing the vena contracta area more than what normally occurs during flow through an orifice in a flat plate. In contrast, but to a lesser extent, $C_d$ increases as the water surface approaches the top of the orifice. Best fit equations to model $C_d$ were developed to better estimate flow, with the most applicable having an $R^2$ of 88.8% and a root-mean squared error of 0.028. Orifice elevation above the tank floor was measured and initially modeled but was ultimately not included in the $C_d$ model because the $R^2$ improved by only 0.6%.

Labyrinth weirs with multiple crest elevations (i.e., staged labyrinth weirs) can be used in spillway design to confine base flows to a section of the crest and/or satisfy discharge hydrograph requirements. However, inadequate hydraulic design information is available specific to staged labyrinth weirs. In this study, the flow characteristics of various staged labyrinth weir configurations (laboratory-scale) were tested. Observations of staged labyrinth weir flow characteristics are presented. The influences of the lower stage length, depth, and location on discharge were studied and head-discharge relationships were experimentally determined. The accuracy of a head-discharge predictive technique on the basis of superposition and traditional labyrinth weir empirical data was also evaluated. Relative to the experimental results, the superposition technique estimations were generally within $\pm 5\%$ for all configurations tested except at lower headwater depths, where maximum estimation errors occurred (maximum of 15%). When discharge was limited to the lower stage weir segment, the predictive discharge errors were up to 20% for some notch configurations. This indicates the discharge of the lower stage segment is location-specific because of the complexity of the labyrinth weir geometry.

Root water uptake is often modeled as a sink term in the Richards equation to account for evapotranspiration losses. The sink term is assumed to be a power law function in the nonlinear root uptake model (O-R model) to account for the decrease in root density and water uptake with depth. In order to extend the O-R model to other crops without requiring detailed and time-consuming experiments, two important dimensionless numbers (specific transpiration, $T_s$, and specific root water uptake, $T^{*}$) are identified based on readily available plant parameters. Data for determining the relationship between these numbers are obtained by minimizing the deviations between the field-observed moisture depletions of 28 crops reported in literature and the numerically simulated soil moisture depletions. Field experiments on three Indian crops—maize, Indian mustard, and wheat—are used for validation of the proposed empirical relationship. This relationship shows promise for use in the O-R model for root water uptake for a variety of crops.