Irrigation Application Uniformity Research Articles

Does investment in irrigation technology necessarily generate rebound effects? A simulation analysis based on an agro-economic model

Investing in more efficient irrigation technology is usually regarded as a means to reduce the use of water by irrigated agriculture. However, some authors report that the introduction of irrigation systems that apply water more uniformly may actually increase water catchment depletion: the so-called ‘rebound effect’. In this paper a simple model that combines irrigation, agronomic, and microeconomic concepts is used to systematically analyze the conditions under which improved irrigation application uniformity may lead to increased water use and/or consumption. The analysis is illustrated with examples from the experience with irrigation modernization in Spain. Water demand (the value of marginal water productivity) becomes inelastic as the irrigation application uniformity increases. The increase in water depletion due to the introduction of more uniform irrigation systems is insignificant if land is limited and farmers optimize their profit. If land is not a limiting factor, new water abstractions are likely to occur, potentially leading to a vicious circle in which irrigated land expands while water resources become overexploited. This cycle tends to slow down as irrigation application uniformity increases. More accurate water accounting is suggested as an instrument for controlling water depletion complementary to investing in irrigation efficiency when water conservation at the basin scale is the main objective.

Assessing whole-field sprinkler irrigation application uniformity

In order to assess whole-field sprinkler irriga- tion uniformity, an experiment was conducted to obtain water distribution profiles at 23 different pressures for each of five different sprinklers: Nelson R33, Nelson R33LP, Nelson R33 with road guard, Nelson R33LP with road guard, and Rainbird Mini Paw/LG-3. A mathematical model was developed to account for pressure variation throughout a fixed sprinkler system on a 10-ha field and to evaluate sprinkler irrigation uniformity for the whole field using interpolated water distribution profiles from the experimental data. The relationships between irrigation application uniformity and sprinkler pressure, sprinkler spacing, pressure variation, sprinkler type, and field topography were studied using the model. The results show that the coefficient of uniformity, CU, decreases rapidly when the pressure is below the low end of the manufac- turer-recommended range; however, CU changes very little with pressure within the manufacturer-recommended range. The system application uniformity, CUsys, is usually less when pressure variations at different locations in a field are considered, and a simple previously published equation to predict CUsys is shown to closely approximate the CU from a more stringent calculation method. It was found that the impact of pressure variation (within the tested ranges) on application uniformity is less than that of the sprinkler spacing. Also, the effect of field topography on sprinkler application uniformity is relatively small for the cases tested herein.

Relationships between common irrigation application uniformity indicators

The coefficient of uniformity, CU, and the distribution uniformity, DU, are perhaps the two most common indicators of irrigation application uniformity, especially for pressurized irrigation methods. The magnitude of CU is usually greater than that of DU, but this is not the case for all data sets, as has been observed in practice by irrigation engineers and researchers. This paper describes the conditions under which CU > DU, and vice versa, proving that either situation can occur in practice. A comparison of some alternative measures of irrigation application uniformity is also compared using two data sets from agricultural sprinklers operating at different pressures.

Inhibiting Water Infiltration into Soils with Cross‐linked Polyacrylamide: Seepage Reduction for Irrigated Agriculture

High water infiltration rates in unlined canals, reservoirs, and the inflow end of furrows relative to outflow ends result in excessive seepage losses and reduced furrow irrigation application uniformity. This study evaluated the use of cross‐linked, anionic, polyacrylamide hydrogel (XPAM), a water‐absorbing, swellable polymer solid, for reducing infiltration and seepage losses though soil. Experiments 1 and 2 measured the influence of soil treatments on seepage rate in soil columns under constant‐head conditions: Exp. 1 treated five soils with 0, 2.5, 5, and 10 g kg−1 XPAM; Exp. 2 applied combined XPAM (0–5 g kg−1) and NaCl (0–5.1 g kg−1) treatments to a silt loam soil, and separately tested the effect of XPAM granule size and treated soil layer thickness on seepage rate. In Exp. 3, a miniflume was used to determine how a 5‐mm‐thick, XPAM‐treated (0–5 g kg−1) soil layer at the inflow end of the “minifurrow” influenced water distribution. The 21‐h seepage rates of all soils except the loamy sand decreased curvilinearly with increasing XPAM rate, with maximum reductions of 87 to 94 0.000000or 5 and 10 g kg−1 XPAM rates, relative to controls. The <300‐μm‐diam. XPAM granules were significantly more effective than the coarser grained XPAM for reducing seepage, and reducing the thickness of the treated soil layer from 71 to 24 mm had no significant effect on the seepage reduction obtained with XPAM. The 5 g kg−1 XPAM treatment applied to inflow‐end miniflume soils significantly decreased the “furrow‐stream” advance period and reversed the infiltration patterns observed in miniflumes, relative to controls. These XPAM treatments could potentially be used to increase the uniformity of furrow water applications and reduce seepage from unlined irrigation ponds and canals.

Command Area Water Demands. I: Validation and Calibration of UCA Model

Simulation of aggregate water demands by areas commanded by canal and lateral turnouts is an important input to the operation and management of the irrigation delivery system. A computer simulation model to predict these command area water‐demand functions was developed and reported by Keller in 1987. To test the validity of the model, data from an existing irrigation project in the Delta, Utah, area were used to simulate the aggregate irrigation demand of five turnouts operated on a demand basis. During the model calibration process, the influence of three important variables in the operation stage on the prediction of demand was identified using the cumulative water‐demand curve. The key variables were: (1) Management‐allowable depletion (MAD); (2) irrigation application uniformity (IAU); and (3) initial soil moisture (ISM).

Command Area Water Demands. II: Water‐Demand Function

The cumulative water demand (CWD) curve was used for validating and calibrating the unit command area (UCA) model, which predicts the aggregate water demands of individual command areas for the entire crop season. During the model calibration process, as presented in the companion paper, three important variables in the operation stage on the prediction of demand were found to be dominant in determining the CWD curve profile. The logistic function with three parameters, which is a particular form of the exponential-type functions, was adopted for fitting the CWD curve. High correlations were observed between the individual curve parameters and the field variables. Three parameters are management-allowable depletion (MAD), irrigation application uniformity (IAU), and initial soil moisture (ISM). Effectiveness of the equation developed for a particular UCA using the regression analysis was successfully demonstrated, and applicability for some other UCAs in the same water system was also recognized.

Trickle Irrigation — Application Uniformity From Simple Emitters

Trickle Irrigation — Application Uniformity From Simple Emitters