Simulating Annual Irrigation Requirement for Citrus on Excessively Drained Soils

A water use simulation for citrus (Citrus sinensis) was used to estimate the effects of climate, soil-available water, rooting depth, allowable depletion of available water, and partial coverage irrigation on the annual irrigation requirements. The soil in the study was excessively drained Candler sand (hyperthermic, uncoated Typic Quartzipsamments) of the Central Florida Ridge. Variation of annual rainfall from 667 to 1827 mm had a relatively small impact on annual irrigation requirements. Soil-available water, depth of root zone, and allowable depletion of available water all affected irrigation management and the number of irrigations annually. Simulated annual irrigation requirements varied over a wide range depending on the allowable depletion of soil-available water, irrigation depth, and the fraction of the land area that is irrigated. Effective rain estimated by the TR21 method during months of high rainfall was higher than estimates by the water budget. Monthly irrigation requirements varied seasonally and peaked in normally dry spring months of April and May. The irrigation simulation is a useful tool for examining the range of management strategies that can be considered for citrus. Citrus grown on the excessively drained soils of the Central Florida Ridge (CFR) require irrigation for optimal production (Koo, 1963). The amount of irrigation needed varies with the amount and distribution of rainfall, soil water availability, rooting depth, management of irrigation, and the percent of the land area that is irrigated. Available water is the difference between the soil field capacity (FC) and permanent wilting point, and is dependent on the soil particle characteristics. Understanding the relative importance of these factors on irrigation requirements of citrus could improve irrigation efficiency and provide a basis for allocation of water for citrus production. The allowable depletion of available water (ADAW) is the percent of available soil water that is depleted before irrigation is scheduled (Smajstrla et al., 1987). The ADAW used for a specific crop depends on the sensitivity of the crop to water stress at various stages of growth. It is important to set the ADAW at a soil water depletion level that will provide the amount of water needed for optimal production. It is common practice with citrus production to maintain more water in the soil during the spring period of flowering and fruit set and reduce irrigation during the rest of the year. The recommendation for irrigating at 33.3% ADAW in the spring and 66.7% ADAW the rest of the year (33:67 ADAW) was based on overhead sprinkler irrigation systems that covered 100% of the land area (Koo, 1963). The current recommendation for low-volume irrigation systems that provide less than full coverage is to irrigate at 25% depletion in the spring and at 50% the balance of the year (Parsons et al., 2000). When the ADAW is reached, irrigation is applied to bring the root zone back to FC. Most citrus on the CFR is irrigated with microsprinklers that cover only part of the total land area. The evaporative demand for water is the same for citrus groves receiving partialor full-coverage irrigation (Smajstrla et al., 1987). During dry weather, the soil water in the irrigated zone is maintained at an optimal level, but the soil water content of the nonirrigated area declines during extended periods without rain. The nonirrigated area supplies significant amounts of water for several days after rain, but this rapidly drops to a trace during dry periods. The rate of water uptake in the wetted area is unaffected by the water content of the nonwetted area (Morgan et al., 2006). Increasing either the frequency or duration of water applied to the irrigated zone in an attempt to compensate for the lack of full coverage will exceed the water storage capacity of the wetted area (Smajstrla et al., 1987). For example, calculating the irrigation requirements for a full coverage site and applying this amount of water to a site irrigating only 50% of the land area would exceed the water-holding ability of the soil in the irrigated area and result in water loss as a result of deep percolation (DP). There is very little horizontal movement of water in the soils of the CFR (Fares et al., 2001), which further limits the amount of water that can be applied. The total amount of water that can be effectively used in partial-coverage irrigation is limited by the fraction of the land that is irrigated and the fraction of tree root length density (RLD) in the wetted zone. The RLD drops rapidly with distance from the trunk (Morgan et al., 2006). Therefore, the rate of water uptake in the irrigated area is proportional to the fraction of the total root length in the wetted area. Thus, irrigation amount is a function of the area and thereby the root mass within the irrigated area. This limitation is of critical importance during Florida’s typical dry spring season when water uptake from roots in the nonirrigated area is much reduced. Irrigation expressed on the land area (ILA) basis is used in reporting and permitting water use, but irrigation management is based on the wetted area (IWA). The ILA is lower than IWA for partial-coverage irrigation because the entire land area is not being irrigated. Water for citrus irrigation in Florida is regulated by several water management districts and the method of allocation is a concern of growers. Most models used by the water management districts (Jacobs and Satti, 2001) calculate irrigation requirement as the difference between estimated monthly crop evapotranspiration (ETc) and effective rainfall (ER). ER is the portion of rainfall that plants can use to meet their need for water and is an important concept in water allocation policy. The TR21 method developed by the U.S. Department of Agriculture is a general method for estimating monthly ER and predicting irrigation requirements as described by (Obreza and Pitts 2002). Their recent report on ER for citrus on poorly drained shallow soils of southwest Florida compares irrigation requirements based on a daily water budget with the TR21 model. They found the water budget to be effective and obtained good agreement between the water budget and the TR21 method. However, ER calculated by the TR21 method was higher than the water budget calculation during months of high rainfall. Recent research on water use by citrus growing on the CFR provided a basis for developing a water budget (Morgan et al., 2006). A program was written to simulate irrigation management based on a water budget and to determine the sensitivity of annual Received for publication 6 Dec. 2005. Accepted for publication 2 June 2006. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 41(6) OCTOBER 2006 1487 irrigation requirements to soil, climatic, and management factors. The goal was to determine the relative importance of factors affecting annual irrigation. Materials and Methods A Hamlin orange grove in Lake County, Fla., was used as a model for this study. It is located at latitude 28 28#20# N, longitude 81 38#50# W and was planted in 1987 with Hamlin orange trees on Carrizo citrange (Citrus sinensis x Poncirus trifoliata) rootstock using a 4.56 · 7.60-m spacing. Each tree was irrigated with one microsprinkler, which provided 3 mm per h to a wetted area diameter of 4.9 m. The irrigated area covered 52% of the area allocated for each tree. The soil was a Candler fine sand (hyperthermic, Typic Quartzipsamments) with total available water-holding capacity of 0.06 mm.mm. The water budget simulation was run using the soil characteristics of this specific site and historical climate records for this location. Partial-coverage irrigation requires maintaining a water budget for both the irrigated and nonirrigated portion of the space allocated for the tree. The water budget was based on the classic publication by (Smajstrla et al. 1987) and modified with new information developed by (Morgan et al. 2006). The budget tracked daily gain and loss of water and simulated irrigation whenever the ADAW was reached. It also estimated water loss from DP whenever FC was exceeded. Partial-coverage irrigation increases the complexity of a water budget. The fraction of the land area irrigated and the fraction of the RLD of the fibrous root system in the wetted area affect the irrigation requirements. Water from both the irrigated and nonirrigated zones contributes to the water available to the tree. The ETc for both the irrigated and nonirrigated areas was calculated from reference Penman ET (ETo) adjusted by a seasonal crop coefficient (Kc) and a stress coefficient (Ks). The stress coefficient reduced ETc as the available water decreased. Values for Kc as a function of day of year (Julian day) and Ks have been determined for CFR environmental conditions and soil characteristics (Morgan et al., 2006). The rate of water uptake from the irrigated area depends on the daily ETc and on the percent depletion of soil available water (Morgan et al., 2006). It is therefore possible to estimate daily water use and to track the percent depletion of available water for the both the irrigated and nonirrigated zones based on proportions of root mass in each zone. Accuracy is enhanced because the system is automatically reset to FC whenever irrigation or rain events exceed FC. The water budget was derived from the following equation: SWCtoday = SWCyesterday + rain + IWA – DP RO – (ETo * Kc * Ks). SWC Soil water content in the irrigated zone (mm) Rain Daily rainfall (mm) IWA Daily irrigation based on wetted area (mm) DP Water loss from deep percolation (mm) when SWC > FC RO Daily runoff assumed 0 as a result of high soil porosity (mm) ETo Reference Penman ETo (mm) Kc Crop coefficient seasonal adjustment Ks Stress index reduces ETc as soil dries. Simulations over 5 year included years of low and high rainfall, variation in soil-available water, depth of irrigated zone, percent of land area irrigated, and ADAWs of 25:25, 25:50, 25:75, 50:50, and 50:75. The first number is the percent ADAW for 15 Feb. to 15 June, which is the critical flower and fruit set period. The second value is ADAW for the rest of the year. Estimated daily ETo values (Smajstrla, 1993) for this grove site were determined using historic daily weather data obtained Table 1. The effects of year, rain, ETo, ETc, and seasonal ADAW on the irrigation requirements, NI, and DP for citrus growing on the CFR. DR ADAW IWA ILA Rain DP ETo ETc ER Year (mm) (%) (mm) (mm) NI (mm) (mm) (mm) (mm) (mm) 2000 457 25:25 772 414 93 667 425 119