DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION Effective Rainfall in Poorly Drained Microirrigated Citrus Orchards

ment, TR-21 was generally recognized to be applicable to areas receiving low intensity rainfall, and to soils that Effective rainfall (ER) is the portion of total rainfall that plants have high infiltration rates (Dastane, 1974). The data use to help meet their consumptive water requirements, and is an important component of water resource budgeting for irrigation. The used to develop the ER equation in TR-21 were obtained USDA’s Technical Release no. 21 (TR-21) is used to predict ER and before the development of microirrigation. Since only irrigation requirements for south Florida citrus, but its accuracy is in part of the field surface area is wetted with microirrigaquestion due to high-intensity rainfall, poorly drained soils, and partial tion, this could seriously affect estimates of the irrigairrigation coverage in microirrigated orchards. We evaluated the caltion requirement for microirrigated crops. Most Florida culation of ER by TR-21 under these conditions by monitoring rainfall, citrus is microirrigated with drip or microsprinkler irrigation, water table depth, evapotranspiration (ET), and soil water irrigation systems. The USDA-NRCS has cautioned uscontent inside and outside of the microirrigation-wetted pattern in ers regarding the limitations of TR-21 (USDA, 1993). four orchards for a total of 83 site-months. We developed a soil water Although annual rainfall for peninsular Florida usubudget to calculate daily water table upflux, root zone water content, water used, and ER separately for the irrigated and nonirrigated root ally exceeds 1300 mm, the majority occurs in summer. zones. Water budget ER calculated by site ranged between 3.3% Additionally, the soils on which most citrus is grown in and 18.2% of TR-21 ER, with a mean of 10%. A linear correlation south Florida are sandy with low water-holding capacity. between water budget ER and TR-21 ER using pooled data from all The uneven rainfall distribution and sandy soils are the four sites yielded the equation: Water Budget ER (mm) 0.79 [TRprimary factors that necessitate irrigation for commer21 ER (mm)] 17.7, r 0.84. A hypothetical ER comparison using cial Florida citrus production. Most south Florida soils 30-yr mean rainfall and ET data showed that annual ER calculated are poorly drained in an undisturbed state. Since citrus by TR-21 amounted to 673 mm, while water budget ER totaled 744 roots are sensitive to excess water (Calvert et al., 1967), mm, or 10.5%. We suggest that the TR-21 method has the level of water management must include drainage as well as accuracy needed to allocate water for microirrigated citrus on poorly irrigation. Drainage is typically a combination of beddrained south Florida soils. ding (0.5to 1-m high double-row beds) and collector ditches from which water is removed either by pumping or gravity flow. T portion of total rainfall that plants use to help The flux of water supplied by upward capillary movemeet their consumptive water requirements is ment from a deep water table is small or nonexistent, termed ER (USDA, 1970). Effective rainfall is often an so it is not taken into account in the TR-21 procedure. important component of irrigation requirement estiHowever, a water table is often present close enough to mates. Technical Release no. 21 has been used worldthe tree root zone in poorly drained soils to significantly wide to predict irrigation requirements (USDA, 1970). augment soil water available for root uptake, reducing However, developments since 1970 have provided betthe irrigation requirement (Obreza and Admire, 1985). ter analytical tools and data for more precise estimates In addition, the presence of a water table limits the of water requirements (Martin et al., 1993). Additionpenetration of citrus roots to ≈0.45 m (Calvert et al., ally, researchers have identified specific problems when 1967), which reduces the potential for ER. using TR-21 under the high-intensity rainfall and poorly The relationship between irrigation requirement (IR) drained soil conditions typical of south Florida (Uribe and the other components of the water budget is: et al., 1995). A comparison of the USDA-SCS 1967 ER estimation IR ETC UF ER, [1] method (later known as the TR-21 method) with a more precise water balance model indicated that this method where ETC is citrus evapotranspiration, is change in overpredicted ER for a slowly permeable, poorly drained root zone soil water stored, and UF is upward flux from soil (Patwardhan et al., 1990). Soon after its developthe water table. All water budget components have the same units, volume per unit area, expressed as depth. The value of ETC can be estimated from daily referT.A. Obreza, Univ. of Florida, IFAS, Southwest Florida Research and Education Center, 2686 State Road 29 North, Immokalee, FL 34142-9515; D.J. Pitts, USDA-NRCS, 1902 Fox Dr., Champaign, IL Abbreviations: DP, deep percolation; ER, effective rainfall; ET, 61820. Florida Agric. Exp. Stn. Journal Series no. R-07737. Received evapotranspiration; IR, irrigation requirement; SF, soil water storage 24 Aug. 2000. *Corresponding author ([email protected]). factor; TR-21, SWFREC, Southwest Florida Research and Education Center; USDA Technical Release no. 21. Published in Soil Sci. Soc. Am. J. 66:212–221 (2002).