Management Allowable Depletion Research Articles

Soybean yield response to managed depletion irrigation regimes in a Mid-South silt loam soil

Soybean irrigation experiments were conducted for five years (2013–2017) on silt loam soil in Jackson, TN, a sub-humid region with high and variable precipitation. The purpose of the experiment was to investigate managed depletion irrigation (MDI) regimes in soybean to obtain the highest yield with minimal water applied on high water-holding capacity soil. Another goal was to find the best use of soil matric potential sensors and water balances to achieve MDI goals. Irrigation supplemented rainfall at a rate of 1.27, 2.54, and 3.81 cm wk−1 starting from first bloom (R1), first pod (R3), and first seed (R5) stages and these treatments were compared against a rainfed control. The results lead to the recommendation that delaying irrigation to R5 in most years and R3 in dry years produced yields at the highest significance level. The connection between irrigation rate and yield fluctuates yearly, but generally, higher rates result in greater crop yields, necessitating a high irrigation rate and rainfall for managed depletion after initiation. The water balance method best emulated optimum MDI results when irrigation was delayed until the soil moisture depletion reached the management allowable depletion (MAD) and then was followed by irrigation that maintained soil water at the MAD level. The irrigation trigger points were initially lowered for matric potential sensors to avoid excess watering in early growth, transitioning to higher levels for optimal yield later in the R3 stage. These results may be applicable in other regions with high water holding capacity soil and moderate to high rainfall rates during the growing season.

Exploring deficit irrigation as a water conservation strategy: Insights from field experiments and model simulation

Deficit irrigation has emerged as a viable approach to increase agricultural water productivity. The objectives of this study were to 1) develop water conservation strategies by investigating responses of green bean (Phaseolus vulgaris) and sweet corn (Zea mays var. saccharata) to four irrigation treatments and 2) evaluate the performance of the Decision Support System for Agrotechnology Transfer (DSSAT) model in simulating growth and yield responses of green bean and sweet corn under four irrigation treatments. A field experiment was conducted using 32 experimental plots, established under a linear move sprinkler irrigation system equipped with variable rate irrigation (VRI) technology, at the Tropical Research and Education Center (TREC), Homestead for two cropping seasons between November and March in 2020–2021 and 2021–2022. The experiment involved four irrigation treatments: full irrigation (FI) (T4), and three levels of deficit irrigation, 75% FI (T3), 50% FI (T2), and 25% FI (T1) with four replications arranged in a completely randomized block design (RCBD) for each crop. The average soil moisture content of the full irrigation treatments was maintained above the management allowable depletion (MAD). Field data collection included various parameters of the crops including plant phenology, plant height, canopy width, fresh and dry biomass, and yield. Results confirmed that all three deficit irrigation treatments did not significantly affect the yield and yield components of the two crops during both seasons compared to the T4. This finding revealed that the highest crop water productivity was achieved from T1, 38.3 and 41.4 kg m−3 for green bean and 54 and 53 kg m−3 for sweet corn during the 2020–2021 and 2021–2022 seasons and up to 75% irrigation water saving could be achieved without affecting plant growth and yield. The DSSAT model performed well in simulating yield and yield components for the two crops in response to the four irrigation treatments. Model evaluation results showed that the minimum mean absolute error (MAE) for green bean fresh pod yield simulation was 2.1 Mg ha−1 with an index of agreement (d-Stat) of 0.6, while for biomass, it was 0.24 Mg ha−1 with a d-Stat of 1. Similarly, for sweet corn, the model simulated fresh cob yield with a minimum MAE of 6.51 Mg ha−1 and d-Stat of 0.9 and biomass with MAE of 1.63 Mg ha−1 and d-Stat of 0.9. Overall, implementing deficit irrigation in green bean and sweet corn fields could result in considerable water savings while achieving acceptable crop yield. The DSSAT model could be also a useful tool to simulate green bean and sweet corn responses to different irrigation scenarios and aid water management decisions under South Florida conditions.

Evapotranspiration Rates of Three Sweet Corn Cultivars under Different Irrigation Levels

Understanding plants’ response to different irrigation levels is essential for developing effective irrigation scheduling practices that conserve water without affecting plant growth and yield. The objective of this study was to evaluate the responses of three sweet corn (Zea mays var. saccharata) cultivars 1170, 8021, and Battalion under three irrigation levels (50%, 75%, and 100%). Irrigation treatments were based on soil moisture management allowable depletion. Replicated trials were conducted, in an open field using 1-gal containers, at the Tropical Research and Education Center, Homestead, FL. A drip system with microsprinklers was used for irrigation. Daily crop evapotranspiration (ETc) rates were measured using a digital scale based on differences in weights of soil containers and plants. Reference evapotranspiration (ETo) was calculated using the FAO-Penman-Monteith equation. Crop-coefficient (Kc) values for the three cultivars were calculated from measured ETc and calculated ETo. In addition, leaf area, stomatal conductance, and fresh biomass were measured. Total irrigation amounts corresponding to the 50%, 75%, and 100% treatments were 116, 162, and 216 mm, and total ETc values were 128, 157, and 170 mm, respectively. The two deficit irrigation treatments (50% and 75%) resulted in a reduction of ETc for the three cultivars compared with the 100% irrigation treatments. Results also showed that under 75% and 100% treatments, Kc values were usually greater than 1 for the three cultivars and reached as high as 1.5. Additionally, leaf area and fresh biomass weight in the 50% treatment were mostly lower than in the 75% or 100% treatments.

Efekti različitih tretmana zemljišta na prinos useva u Jugoistočnoj Nigeriji

In this study, a PVC drip irrigation system was designed to investigate the effect of different soil treatments on crop yield, using Oba Super 13 maize variety as test crop, on three major plots, each representing a different tillage method. Each plot has three levels of each of the three soil treatments involved including irrigation deficit, tillage method and NPK Application rate, totaling 27 subplots. The three levels of irrigation treatments were 50%, 30% and 10% management allowable depletion levels; tillage treatments were conventional tillage, conservative tillage and no tillage methods, while the NPK application treatments were 400 kg/ha, 500 kg/ha and 600 kg/ha rates, and experimentally designed using the Central Composite Design (CCD) in Design Expert 11 software. The crop yield for all the subplots were determined, and maximum crop yield of 2540 kg/ha was obtained at conservative tillage with 10%MAD, and 600 kg/ha NPK application rate, while minimum tillage of 1234.67 kg/ha was obtained at no tillage, 50%MAD and 400 kg/ha NPK application rate. Controllable variables were optimized using response surface methodology (RSM) with crop yield for all the subplots. The optimum values based on the run gave irrigation deficit as 11.594%, NPK Application rate as 596.406 kg/ha, best tillage method as conservative tillage, crop yield of 2543.589 kg/ha. The highest maize yield was obtained in conservative tillage and the results confirm the viability of obtaining high yield in the study area using drip irrigation system during the dry season.

Growth and nutrient uptake of haloxylon and atriplex as affected by mycorrhizal symbiosis under combined drought and salinity stresses

This study was conducted to determine the effect of arbuscular mycorrhizal fungi (AMF) symbiosis on plant growth and nutrient uptake under combined drought and salinity stresses. A pot experiment was carried out with a factorial arrangement of treatments in a completely randomized design with three replications. Experimental treatments included two plant types ( Atriplex canescens and Haloxylon ammodendron) with three levels of inoculation of fungal species ( Funneliformis geosporus, Funneliformis mosseae, and control), two levels of soil salinity stress (7 and 14 dS m–1), and two levels of drought stress (50% and 80% of management allowable depletion). Vegetative parameters, as well as root N, P, and K concentrations and uptakes, mycorrhizal growth response, mycorrhizal nitrogen response, mycorrhizal phosphorus response, mycorrhizal potassium response, and root colonization were measured. The results showed that the application of AMF increased the plant growth variables such as stem diameter, root length, shoot dry weights, and shoot to root ratio as well as nitrogen and phosphorus uptakes. The application of both AMF types was effective as compared to the control. However, F. mosseae indicated better performance especially, in terms of the effect on plant growth variables. Also, F. mosseae was more effective to relieve stress (i.e., salinity and drought) than F. geosporus. There was a significant difference between plant types and H. ammodendron had better efficiency than A. canescens under drought and salinity stresses. Based on the results, planting of H. ammodendron inoculated with F. mosseae might be recommended for soil conservation in the arid environments.

Design of automatic control system on trickle irrigation for tomato cultivation

The development of technology has had an impact on the progress of irrigation systems, called the intelligent irrigation system 4.0. The objectives of this research are a measurement of Emission Uniformity (EU) and designing automatic irrigation based on soil water sensors. The result of EU value is in the range of 85-90%, which is a good category. EU value can determine whether one emitter can represent the entire emitter in one irrigation system. The principle of the soil water sensor is to read the value of the soil moisture content in the planting medium. The soil water content of 22.4%, which is the field capacity, is used as an upper limit and the soil water content of 21.1%, which is the critical point, is used as the lower limit. Determination of the critical point value is done by using the management allowable depletion (MAD) value of 0.52. The working principle of the control system is to monitor the work of the soil water sensor and become the main data to control valves, pumps, and dosing. The required cost for the automatic system is Rp. 12,626,000, -.

Sustainable irrigation based on co-regulation of soil water supply and atmospheric evaporative demand

Irrigation is an important adaptation to reduce crop yield loss due to water stress from both soil water deficit (low soil moisture) and atmospheric aridity (high vapor pressure deficit, VPD). Traditionally, irrigation has primarily focused on soil water deficit. Observational evidence demonstrates that stomatal conductance is co-regulated by soil moisture and VPD from water supply and demand aspects. Here we use a validated hydraulically-driven ecosystem model to reproduce the co-regulation pattern. Specifically, we propose a plant-centric irrigation scheme considering water supply-demand dynamics (SDD), and compare it with soil-moisture-based irrigation scheme (management allowable depletion, MAD) for continuous maize cropping systems in Nebraska, United States. We find that, under current climate conditions, the plant-centric SDD irrigation scheme combining soil moisture and VPD, could significantly reduce irrigation water use (−24.0%) while maintaining crop yields, and increase economic profits (+11.2%) and irrigation water productivity (+25.2%) compared with MAD, thus SDD could significantly improve water sustainability.

Effect of deficit drip irrigation on yield and water productivity of potato crop

On the eve of changing climate, it is expected that there will be a competition to reallocate water for agricultural, industrial and urban needs in near future. Thus, in view of the increased water scarcity, an adequate management for water application is required to maximize water use and increase yield. In this regard, a field trial comprising four deficit drip irrigation treatments on potato crop under drip irrigation was conducted in filed condition using RCB design with four replications. Cropwat 8.0 program was used to determine the interval and depth of next irrigation at 15, 30, 45 60% management allowable depletion (MAD) of soil moisture. The outcomes indicated that irrigation to potato crop at 30% MAD level gave the highest water productivity (17.28 kg/m3) followed by 60, 15 and 45% MAD levels. Maximum %age of medium sized tuber was achieved by applying irrigation to potato crop at 60% MAD level (65.70) followed by 30% MAD level (65.5), 45% MAD (64.0) and 15% MAD level (60.50). The incidence of scab was found 2.50%, 2.75%, 3.25% and 3.75% in potato tubers at 15, 30, 45 and 60% MAD level, respectively. Thus, growers can save the water by adopting this water saving technique without reduction in tuber yield. For proper adaptation of new technique and practice, common farmer of Pakistan should be educated in term of visits to govt. research institutions and adoptive research farms to understand the right step to words better yield.

Evaluation of Different Moisture Depletion Levels for Water Productivity, Yield and Tuber Size of Potato under Drip Irrigation

A field experiment comprised of four different levels of management allowable depletion (MAD) of soil moisture for next irrigation to potato crop under drip irrigation was conducted during 2017-18 and repeated during 2018-19 at Water Management Research Farm, Renala Khurd, Okara, Pakistan. Cropwat 8.0 program was used to determine the interval and depth of next irrigation at 15, 30, 45 and 60% MAD of soil moisture. The experiments during both the years were organized in RCBD fashion with four replications. Irrigation to potato crop at 30% MAD level depicted the highest water productivity (17.28 kg/m3), followed by 60%, 15% and 45% MAD levels with water productivity values of 16.90, 16.63 and 14.60 kg/m3, respectively. The highest tuber yield was obtained when potato crop was irrigated at 60% MAD level (28.411 tons ha-1), followed by 30% MAD level (27.998 tons ha-1), 15% MAD level (26.736 tons ha-1) and 45% MAD level (24.081 tons ha-1). The maximum %age of medium sized tubers was achieved by applying irrigation to potato crop at 60% MAD level (65.70), followed by 30% MAD level (65.50), 45% MAD level (64.00) and 15% MAD level (60.50). The incidence of scab was found 2.50%, 2.75%, 3.25% and 3.75% in potato tubers at 15, 30, 45 and 60% MAD level, respectively indicating that the incidence of disease increased slightly with increase in MAD level.

Soil water dynamics over 12 seasons on irrigated dry bean–potato–wheat–sugar beet rotations

Dry bean (Phaseolus vulgaris L.), potato (Solanum tuberosum L.), wheat (Triticum aestivum L.), and sugar beet (Beta vulgaris L.) are mainstays of irrigated crop production in southern Alberta. Concerns about soil quality and sustainability instigated a 12 yr (2000–2011) rotation study to compare conventional (CONV) with conservation (CONS) management practices (reduced tillage, narrow-row dry bean, compost addition, and cover cropping). Plant-available water (PAW) was measured using a neutron probe (10–16 count days·season−1, n = 148) on all phases of 4 yr (dry bean–potato–wheat–sugar beet) rotations under CONS and CONV management. A visual monitoring approach was used for irrigation scheduling. For dry bean and sugar beet, management allowable depletion (MAD) was exceeded on only 11%–15% of neutron probe count days over 12 yr. However, MAD was exceeded on 30% of count days for wheat and 43% for potato. Significant crop × management interactions showed that PAW was higher with CONS management most frequently on potato, followed by dry bean, wheat, and sugar beet. This order reflected the prevalence of CONS practices directly impacting each crop. Regression analyses showed that potato, wheat, and sugar beet yield increased significantly as mean growing season water table depth (WTD) increased. This was explained by yield suppression due to excessive soil wetness in seasons with high rainfall and shallow WTD. This study provided comparative soil water dynamics for four major irrigated crops in southern Alberta, over a 12 yr period, which included record high and low growing season precipitation.

Development and Evaluation of a Drip Irrigation System in Southeastern Nigeria

Extreme climatic factors (temperature, precipitation etc) which at times lead to drought and flooding affect crop yield negatively. In this study, a PVC drip irrigation was developed and the irrigation parameters were evaluated in a farmland with three tillage methods (conventional tillage, conservative tillage and no tillage). The irrigation treatments comprised of three levels of irrigation (50% Management allowable depletion, 30% management allowable depletion and 10% management allowable depletion). Different irrigation parameters were evaluated for different crop growth stages, different soil treatments and different soil depths (0-25 cm, 25-50, 50-75 and 75-100 cm depths).
 The field capacity was determined at -0.01MPa, from the result, the field capacity was minimum at no tillage (0.07 cm3/cm3, 0.11 cm3/cm3, 0.12 cm3/cm3, and 0.14 cm3/cm3) for soil depths 0-25 cm, 25-50 cm, 50-75 cm and 75-100 cm respectively, for conservative tillage (0.11 cm3/cm3, 0.11 cm3/cm3, 0.11 cm3/cm3, 0.14 cm3/cm3) for 0-25 cm, 25-50 cm, 50-75 cm and 75-100 cm soil depths respectively and conventional tillage (0.09 cm3/cm3, 0.13 cm3/cm3, 0.15 cm3/cm3, 0.17 cm3/cm3) for 0-25 cm, 25-50 cm, 50-75 cm and 75-100 cm soil depths respectively.
 The permanent wilting point was determined at -1.5MPa, from the result, permanent wilting point increased with increase in soil depth in conventional tillage and no tillage with PWP of 0.01 cm3/cm3, 0.05 cm3/cm3, 0.09 cm3/cm3 and 0.11 cm3/cm3 at 0-25 cm, 25-50 cm, 50-75 cm and 75-100 cm soil depths respectively for conventional tillage and PWP of 0.02 cm3/cm3, 0.05 cm3cm3, 0.05 cm3/cm3 and 0.08 cm3/cm3 at 0-25 cm, 25-50 cm, 50-75 cm and 75-100 cm soil depths respectively for no tillage.
 The average net irrigation was found to be 1.2 cm, 1.56 cm and 1.95 cm for conventional tillage, conservative tillage and no tillage respectively, while the average gross irrigation was found to be 1.7 cm, 1.7 cm and 2.1 cm for conventional tillage, conservative tillage and no tillage respectively.
 Statistical analysis of net and gross irrigation gave a coefficient of determination of 0.99 and p-value at 0.05 was significant with a value of 0.00007.

Pond and Irrigation Model (PIM): a Tool for Simultaneously Evaluating Pond Water Availability and Crop Irrigation Demand

Agricultural ponds are an important alternative source of water for crop irrigation to conserve surface and ground water resources. In recent years more such ponds have been constructed in Mississippi and around the world. There is currently, however, a lack of a tool to simultaneously estimate crop irrigation demand and pond water availability. In this study, a Pond-Irrigation Model (PIM) was developed to meet this need using STELLA (Structural Thinking, Experiential Learning Laboratory with Animation) software. PIM simulated crop land and agricultural pond hydrological processes such as surface runoff, soil drainage, and evapotranspiration as well as crop irrigation demand and pond water availability. More importantly, PIM was able to decide when to conduct crop irrigation based on management allowable depletion (MAD) root zone soil water content and to determine optimal ratios of agricultural pond size to crop land with sufficient pond water available for crop irrigation. As a case demonstration, the model was applied to concomitantly estimate row crops (i.e., corn, cotton, and soybeans) water irrigation demand and pond water availability in a farm located at East-central Mississippi. Simulations revealed that corn used more soil water for growth than soybeans, whereas soybeans needed more irrigation water than corn and occurred due to less rainwater available for soybeans growth. We also found that there was one time for corn, zero time for cotton, and two times for soybeans when the pond water level was drawn to near zero for irrigation from 2005 to 2014. PIM developed in this study is a useful tool for estimating crop irrigation demand and pond water availability simultaneously.

Deficit irrigation improves phenolic content and antioxidant activity in leafy lettuce varieties.

This study investigated the effect of deficit irrigation at three management allowable depletion levels (MAD) 25%, 50%, and 75% on phytochemicals, ascorbic acid, antioxidant activity, glucose, fructose, and fresh and dry mass in two green leafy lettuce cultivars (Lollo Bionda and Vera) at harvest. Genotype response was observed during deficit irrigation on fresh and dry mass at harvest. Vera revealed similar fresh mass at MAD levels 25% and 50%. Chicoric acid, caftaric acid, and chlorogenic and caffeic acid tended to increase at 50% MAD, while kaempferol, quercetin, and myricetin increased at 75% MAD in both cultivars indicating increasing antioxidant properties. Ascorbic acid content declined with increasing MAD levels and no definite trend on β‐carotene content was noted in these cultivars with respect to MAD irrigation. Deficit irrigation at 50% and 70% increased glucose concentration in cv. Lollo Bionda. Deficit irrigation influences the abiotic stress condition that stimulates the biosynthesis of phytochemicals in plants and improves crop quality. However, deficit irrigation can affect marketable crop yield. Based on findings of this study, the lettuce cv. Vera can be recommended as a suitable cultivar for deficit irrigation (at MAD 50% levels) for improving dietary phytochemicals and crop quality without compromising fresh mass for marketing.

Using a Crop Simulation Model to Understand the Impact of Risk Aversion on Optimal Irrigation Management

Abstract. We studied optimal irrigation management by risk-averse farmers with different soil types under limited well capacity. Our modeling framework allowed us to assess the optimal adjustment along the intensive margins (i.e., changes in seasonal irrigation depth) and along the extensive margins (i.e., changes in irrigated area). Our empirical application uses AquaCrop to simulate corn yields with historical weather in southwest Kansas under a large number of potential irrigation strategies. We show that risk aversion significantly increases total water use, especially for low and medium well capacities. While farmers decreased irrigated area due to risk aversion, the increase in water use occurred because it was optimal to increase the seasonal irrigation depth to reduce production risk. The increase in seasonal irrigation depth arises mostly from reduced management allowable depletion (MAD) levels in the initial crop growth stages of corn. Counterintuitively, risk aversion had a smaller impact on water use for a soil with a smaller soil water holding capacity. This result arises because optimal irrigation under risk neutrality is larger for soils with a smaller water holding capacity. Our results highlight the importance of accounting for risk aversion when estimating the optimal irrigation management strategy and show that the impact of risk aversion differs significantly by well capacity and soil type. Keywords: AquaCrop, Irrigation, Risk, Well capacity.

Development and integration of soil moisture sensor with drip system for precise irrigation scheduling through mobile phone

Soil moisture sensor is an instrument for quick measurements of soil moisture content in the crop root zone on real time basis. The main objective of this research was development and evaluation of an indigenous sensor for precise irrigation scheduling. The various parts of sensor developed were ceramic cup, acrylic pipe, level sensor, tee, reducer, gland, cork, and end cap. The designed system was successfully tested on okra crop and calibrated with frequency domain reflectometry (FDR) by three methods of irrigation, i.e. check basin, furrow and drip, respectively. The average depth of water depletion in modified tensiometer by these methods was 27 to 35 cm at 50% management allowable depletion (MAD) of field capacity. This depth was useful for the level sensor to be installed inside modified tensiometer for real time irrigation scheduling. The correlation coefficient (R2) between soil moisture content obtained from the developed sensor and FDR was 0.963. Sensor network was integrated with global system for mobile communication (GSM), short message service (SMS) and drip head work to develop an automated irrigation system. This would enable farmers to effectively monitor and control water application in the field by sending command through SMS and receiving pumping status through the mobile phone.

Development of a Simple Irrigation Scheduling Calendar for Mesilla Valley Pecan Growers

For farmers to accurately schedule future water delivery for irrigations, a prediction method based on time-series measurements of soil moisture depletion and climate-based indicators of evaporative demand is needed. Yet, numerous reports indicate that field instruments requiring high in-season labor input are not likely to be used by farmers. In New Mexico, pecan (Carya illinoensis) farmers in the Mesilla Valley have been reluctant to adopt new soil-based or climate-based irrigation scheduling technologies. In response to low adoption rates, we have developed a simple, practical irrigation scheduling tool specifically for flood-irrigated pecan production. The information presented in the tool was derived using 14 years of archived climate data and model-simulated consumptive water use. Using this device, farmers can estimate the time interval between their previous and the next irrigation for any date in the growing season, in a range of representative soil types. An accompanying metric for extending irrigation intervals based on field-scale rainfall accumulation was also developed. In modeled simulations, irrigations scheduled with the tool while using the rainfall rule were within 3 days of the model-predicted irrigation dates in silty clay loam and loam soil, and less than 2 days in sandy loam and sand soil. The simulations also indicated that irrigations scheduled with the tool resulted in less than 1% reduction in maximum annual consumptive water use, and the overall averaged soil moisture depletion was 45.14% with an 18.1% cv, relative to a target management allowable depletion of 45%. Our long-term objective is that farmers using this tool will better understand the relationships between seasonal climate variation and irrigation scheduling, and will seek real-time evapotranspiration information currently available from local internet resources.

Assessment of Microirrigation for Sugarbeet Production

ABSTRACT Sugarbeet (SB) is a major value-crop in many furrow (Fu) irrigated areas of Wyoming where nitrate (NO3) contamination prevails. Microirrigation (M) may be an option for managing such pollution while sustaining SB production. With the goal of evaluating efficacy of M for SB production, in comparison to Fu practice, studies were conducted through field experiments, computer simulations, and economic assessments. The field study with SB consisted of a Fu and three M treatments (M1, M2, and M3 representing a management allowable depletion factor of 0.20, 0.35, and 0.50, respectively) and three fertilizer doses. Water use, post-harvest soil NO3 concentrations, SB yields, and water percolation were measured. Computer simulations were performed using a transport and irrigation model to predict soil NO3 concentrations and the results were compared with the experimental data. The economic feasibility of adopting M for 10 to 40 ha area was evaluated using a capital budgeting analysis. Results of field studies indicated lower water use, higher soil NO3 levels, less water percolation and greater SB yields with M than under Fu practice. The yields increased as management allowable depletion factor and fertilizer rate augmented. Computer simulations predicted that post-harvest soil NO3 concentrations were higher when SB were irrigated with M rather than Fu practice. Simulation results were comparable with the field data. The economic analysis indicated that irrigation costs decreased and SB returns increased with M practice. Results of capital budgeting showed that adoption of M in Wyoming was feasible for small-scale SB production. The profitability of the M system was dependent on the irrigation regime and the conversion area.

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

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.

Optimization of furrow irrigation schedules, designs and net return to water

A seasonal furrow irrigation model consisting of irrigation scheduling and kinematic-wave-based hydraulic submodels was modified to incorporate an economic optimization submodel. The model used a systematic simulation technique to optimize furrow irrigation schedules and designs assuming 80% irrigation adequacy at cutoff time. The irrigation schedules and designs were optimized for the homogeneous and heterogeneous infiltration under the mean and observed ET o (grass reference crop ET) conditions. The optimal management allowable depletion (MAD) level changed with the variation in ET o condition, and with the consideration of spatial and temporal (seasonal) variability in infiltration characteristics. Irrigation design changed with both infiltration conditions and MAD level. Infiltration variability did not influence the bean yield. However, the return to water decreased when spatial variability in infiltration conditions was considered. Using mean ET o resulted in slightly higher yield and net return to water as compared to using observed ET o. A small variation in daily mean ET o values with respect to daily observed ET o values caused a change in both irrigation schedules and designs. Therefore, mean ET o cannot be used to forecast irrigation schedules and designs at the beginning of crop season. The net return to water increased (1.7 to 3.6%), and the seasonal inflow, losses, and bean yield decreased in the case of variable interval scheduling (holding MAD constant) as compared to the fixed interval scheduling (MAD varies).

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.