E. Playán

Analysis of an irrigation district in northeastern Spain: II. Irrigation evaluation, simulation and scheduling

In the first paper of this series, the Loma de Quinto Irrigation District (LQD) was characterised, and water use was assessed. In this work, the analysis of the LQD is completed with field irrigation evaluations, solid-set sprinkler irrigation simulations and irrigation scheduling for optimal crop yield. The results of the irrigation evaluations indicated that the average Christiansen coefficient of uniformity (CU) for solid-sets, centre-pivots and linear-moves was 68.0, 75.5, and 80.0%, respectively. In solid-sets CU was severely reduced by wind speed. However, in centre-pivots and linear-moves CU was higher in evaluations with wind speeds between 2 and 6 m s-1 than under calm conditions. The evaluation data set was used to validate a ballistic solid-set sprinkler irrigation simulation model. The performance variables used for model validation were CU and the potential application efficiency of the low quarter (PAElq). Both variables were adequately predicted in the range of the observed values. The model was used to extend the evaluation results to all the solid-set plots in the LQD. CU maps were produced for different wind speeds and operating pressures. These maps can be used to identify plots with low irrigation performance. The effect of irrigation scheduling on crop yield and net benefit was analysed using the CropWat simulation model. Simulations of the 1997 irrigation practices performed on a limited number of plots detected a 12% decrease in crop yield due to deficit irrigation and/or large irrigation intervals. The introduction of an optimal irrigation schedule (avoiding yield reductions) would imply increasing the alfalfa seasonal irrigation depth by 101 mm, and applying light, frequent irrigation events. Due to labour scarcity in the LQD, the implementation of the optimal schedule would require a high degree of irrigation automation, which is currently unavailable. Taking into consideration the value of the additional yield and the costs of the extra irrigation water depth and the automation devices, the resulting net benefit would be 50 ha-1. The purpose of this analysis of the LQD is to contribute to the diagnostic analysis phase of an incipient Management Improvement Program at the LQD. In order to complete this phase, an interdisciplinary committee will perform a study not just on irrigation but on a wide scope of irrigated agriculture in the LQD. Author Keywords: Sprinkler irrigation; Irrigation uniformity; Simulation model

Border fertigation: field experiments and a simple model

Abstract Fertilizers are commonly dissolved in border irrigation water. However, there are no available procedures for the proper design and management of surface fertigation. A series of experiments was conducted to determine the uniformity of fertilizer application under varying inflow discharges and fertilizer application times in blocked-end borders. Results from the fertigation evaluations showed that the fertilizer distribution uniformity of the low half (DULH) ranged from 2.9 to 51.6%, while water DULH ranged from 63.5 to 96.9%. A simplified border fertigation model based on one-dimensional convection was formulated and applied to the simulation of the experiments. The model was able to explain 43.8% of the variability in the fertilizer DULH. Application of the model to selected case studies revealed that short-duration applications, such as those resulting from instantaneous release of fertilizer into the irrigation stream, often produce low uniformities. This is particularly true for early applications in blocked-end borders and for early and late applications in level basins. In both irrigation systems, application of fertilizer at a constant rate during the entire irrigation event is frequently the best solution. In the presence of runoff, the model can be used to find a compromise between fertilizer application uniformity and runoff losses. Finally, if large deep percolation losses are expected, the model can identify uniform fertigation options based on late applications of fertilizer.

Simulation of 1D surface and 2D subsurface water flow and nitrate transport in alternate and conventional furrow fertigation

Increasing water and fertilizer productivity stands as a relevant challenge for sustainable agriculture. Alternate furrow irrigation and surface fertigation have long been identified as water and fertilizer conserving techniques in agricultural lands. The objective of this study was to simulate water flow and fertilizer transport in the soil surface and in the soil profile for variable and fixed alternate furrow fertigation and for conventional furrow fertigation. An experimental data set was used to calibrate and validate two simulation models: a 1D surface fertigation model and the 2D subsurface water and solute transfer model HYDRUS-2D. Both models were combined to simulate the fertigation process in furrow irrigation. The surface fertigation model could successfully simulate runoff discharge and nitrate concentration for all irrigation treatments. Six soil hydraulic and solute transport parameters were inversely estimated using the Levenberg–Marquardt optimization technique. The outcome of this process calibrated HYDRUS-2D to the observed field data. HYDRUS-2D was run in validation mode, simulating water content and nitrate concentration in the soil profiles of the wet furrows, ridges and dry furrows at the upstream, middle and downstream parts of the experimental field. This model produced adequate agreement between measured and predicted soil water content and nitrate concentration. The combined model stands as a valuable tool to better design and manage fertigation in alternate and conventional furrow irrigation.

Irrigation Modernization in Spain: Effects on Water Quantity and Quality—A Conceptual Approach

This article analyses the effects of irrigation modernization processes on water quantity and quality, taking the Ebro river basin (NE Spain) as a case study. The objective is to contribute to needed in-depth analysis and discussion regarding the optimization of water use in agriculture. A conceptual approach based on water accounting concepts has been applied. Results show that irrigation modernization linked to an increase in land productivity involves additional water depletion if the location of the irrigated areas and the quality of the irrigation return flows allow their re-use. Additionally, modernization reduces the volume of return flows and pollutant loads and increases the quality of the receiving water bodies. The modernization of water management will be required to maximize economic, social and environmental returns from the investment in new irrigation infrastructure.

Characterizing microtopographical effects on level-basin irrigation performance

Abstract Microtopography has long been recognized as one of the key variables in level-basin irrigation performance, although little effort has been devoted to establish its relevance. In this work, experimental data are used to quantify the influence of microtopography on irrigation performance. An irrigation evaluation was performed on a small level-basin (256 m 2 ) LASER levelled to zero slope. Irrigation depth was gravimetrically measured and estimated at the 49 nodes of a regular network. Data from the irrigation evaluation and a two-dimensional flat-bed model were used to estimate irrigation depth. Irrigation times, soil surface elevation and distance to the inlet were estimated at the same nodes, and a correlation matrix was computed. Results showed that soil surface elevation was highly and significantly correlated with the times of advance (0.725 ∗∗∗ ), recession ( −0.815 ∗∗∗ ) and opportunity ( −0.852 ∗∗∗ ), and with the measured irrigation depth ( −0.583 ∗∗∗ ). Distribution uniformity using soil water measurements was 71.0%. Estimates from the irrigation evaluation and the two-dimensional model were 85.3% and 94.9%, respectively. The irrigation evaluation procedure could explain 30 ∗∗∗ % of the measured variability in irrigation depth. A large part of the unexplained variance in measured irrigation depth seems to be due to the spatial variation of infiltration properties. Predictions by the two-dimensional model were not significantly related to the measured values. A simple method was devised to estimate microtopography-adjusted irrigation performance from the results of a flat bed model and the standard deviation of elevation. Microtopography can have an important effect on level-basin irrigation performance. Models not considering this variable may incur large errors when simulating irrigation performance.

Fertigation in Furrows and Level Furrow Systems. I: Model Description and Numerical Tests

The simulation of fertigation in furrows and level furrow systems faces a number of problems resulting in relevant restrictions to its widespread application. In this paper, a simulation model is proposed that addresses some of these problems by: (1) implementing an infiltration model that adjusts to the variations in wetted perimeter; (2) using a friction model that adjusts to different flows and which uses an absolute roughness parameter; (3) adopting an equation for the estimation of the longitudinal diffusion coefficient; and (4) implementing a second-order TVD numerical scheme and specific treatments for the boundary conditions and the junctions. The properties of the proposed model were demonstrated using three numerical tests focusing on the numerical scheme and the treatments. The application of the model to the simulation of furrows and furrow systems is presented in a companion paper, in which the usefulness of the innovative aspects of the proposed model is demonstrated.

Fertigation in Furrows and Level Furrow Systems. II: Field Experiments, Model Calibration, and Practical Applications

Furrow fertigation can be an interesting practice when compared to traditional overland fertilizer application. In the first paper of this series, a model for furrow fertigation was presented. The simulation model combined overland water flow (Saint-Venant equations), solute transport (advection-dispersion), and infiltration. Particular attention was paid to the treatment of junctions present in level furrow systems. In this paper, the proposed model is validated using five furrow fertigation evaluations differing in irrigation discharge, fertilizer application timing, and furrow geometry. Model parameters for infiltration and roughness were estimated using error minimization techniques. The error norm was based on observed and simulated values of advance time, flow depth, and fertilizer concentration. Model parameters could be adequately predicted from just one discharge experiment, although the use of more experiments resulted in decreased error. The validated model was applied to the simulation of a level furrow system from the literature. The model adequately reproduced irrigation advance and flow depth. Fertigation events differing in application timing were simulated to identify conditions leading to adequate fertilizer uniformity.

Instruments for water quantity and quality management in the agriculture of Aragon

The traditional policy of developing new irrigated areas in Aragon has been changed to irrigation modernization through investments in distribution networks and on-farm irrigation equipment. This new policy creates opportunities to introduce more profitable crops, conserve irrigation water and abate agricultural nonpoint pollution. Several alternatives open to irrigated agriculture are bioethanol and biodiesel technologies (which could provide a support price for grains), the expansion of profitable fruits and vegetables under drip irrigation, and the diversification of water using activities (animal farming, industries, residential areas and sport utilities). Alternative measures to abate agricultural nonpoint pollution are examined in this paper. Modernizing irrigation structures leads to a large reduction of pollution, and introduces reasonable costs to farmers (in terms of their rent). Results also show that water pricing—advocated by the European Water Framework Directive—is a wrong policy in irrigation, because irrigation demand does not respond to prices and also because water pricing is not cost efficient to abate pollution.

Elevation and infiltration in a level basin. II. Impact on soil water and corn yield

Abstract The spatial variability of irrigation water recharge and crop yield is affected by a number of factors. Soil surface elevation, infiltration and soil water MAD are the most relevant related to level-basin irrigation. Measurements of soil water recharge (using a neutron probe) were compared to estimates based on ring infiltrometers and observations of the opportunity time. Estimates of cumulative infiltration (ECI) were obtained, separating the variability of infiltration and opportunity time (largely determined by elevation). Soil surface elevation was correlated with measured recharge, grain yield and total dry matter. A correlation was found between infiltration and the measurements of water recharge. While soil surface elevation can be regarded as a management variable, little can be done to reduce the variability of infiltration. Distribution uniformities from ECI were about 20% higher than those obtained from measurements of water recharge. Seasonal uniformity was only marginally higher than average uniformity, confirming the low random component of water recharge in level-basin irrigation. Deep percolation was more intense in areas with low MAD. This finding emphasizes the relevance of characterizing the variability of soil physical properties in surface irrigation. Extrapolation of the results of this research to field-scale irrigation basins should take into account the methodology used: in particular, the reduced scale of the experimental level basin.

Elevation and infiltration in a level basin. I. Characterizing variability.

Abstract Spatial characterization of soil physical properties could improve the estimation of surface irrigation performance. The aim of this research was to characterize the spatial and time variability of a set of irrigation-related soil properties. The small-scale experimental level-basin (729 m2) was located on an alluvial loam soil. A corn crop was established in the basin and irrigated five times during the season. A detailed survey of the soil properties (generally using a 3 × 3 m network) was performed. Classic statistical and geostatistical tools were used to characterize the variables and their interactions. Semivariograms were validated for the studied variables, except for the clay fraction, the saturated hydraulic conductivity and the infiltration parameters. The resulting geostatistical range was often in the interval of 6–10 m. For the three surveys of soil surface elevation the range was smaller, about 4 m. No correlation was found between saturated hydraulic conductivity and the other soil physical properties. Soil surface elevation showed a high correlation between surveys. After the first irrigation, the standard deviation of elevation increased from an initial 9.6 mm to 20.8 mm. The soil physical parameters were used to map the soil water management allowable depletion. In a companion paper these results are used to explain the spatial variability of corn yield and soil water recharge due to irrigation.