578 SSSAJ: Volume 72: Number 3 • May–June 2008 SOIL PHYSICS Soil Sci. Soc. Am. J. 72:578-585 doi:10.2136/sssaj2007.0167 Received 7 May 2007. *Corresponding author (AFares@hawaii.edu). © Soil Science Society of America 677 S. Segoe Rd. Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. A lthough Florida averages around 1300 mm of rain annu- ally, supplemental irrigation is still required for intensive citrus production because: (i) rainfall is irregularly distributed, with 70% of the annual amount occurring during the sum- mer months; (ii) the water-holding capacity of Florida’s sandy soils (>96% sand) is extremely low; and (iii) intensive citrus production requires maintenance of soil water content near field capacity, especially during the flowering and fruit-setting period, which coincides with the dry period of the year. Citrus crop water use has been calculated for mature (Morgan et al., 2006; Smajstrla et al., 1986) and young (Fares and Alva, 1999, 2000) trees in central Florida. Daily crop evapotranspiration (ET c ) of young citrus trees measured dur- ing the 1996 and 1997 cropping seasons were from 1.9 to 2.0 mm (Fares and Alva, 1999) and from 1.87 to 3.13 mm (Fares and Alva, 2000), respectively, while mature Florida citrus daily ET c ranged from 2.25 to 3.52 mm (Rogers et al., 1983). Effective rainfall is defined as the portion of rainfall that plants use to meet daily evapotranspiration requirements (USDA, 1970). Some of the rainfall may be unavoidably lost due to the combined effect of rainfall intensity, frequency, and amount. Effective rainfall varies along with total rainfall. Water regulating agencies require accurate estimates of crop water bud- get components to fairly allocate irrigation water resources to growers. Effective rainfall (ER) is an important component of the irrigation requirement, IRR (mm), calculations. Irrigation requirements for a particular crop are calculated as follows: ( ) c IRR ET UF ER S Δ = + − + [1] where ΔS (mm) is change in root zone soil water storage and UF (mm) is upward flux from the water table (if present) due to capillary rise. In the deep, well-drained sandy soils of central Florida, UF is negligible. A. Fares* Natural Resour. & Environ. Management Dep. Univ. of Hawaii-Manoa 1910 East-West Rd. Honolulu, HI 96822 A. Dogan Civil Engineering Dep. Suleyman Demirel Univ. Isparta, Turkey F. Abbas Natural Resour. & Environ. Management Dep. Univ. of Hawaii-Manoa 1910 East-West Rd. Honolulu, HI 96822 L. R. Parsons Citrus Research and Education Center Univ. of Florida Lake Alfred, FL 33850 T. A. Obreza Soil and Water Science Dep. Univ. of Florida Gainesville, FL 32611 K. T. Morgan Southwest Florida Res. and Education Center Univ. of Florida Immokalee, FL 34142 Water Balance Components in a Mature Citrus Orchard The low water-holding capacity of sandy soils, together with spatial and temporal variations of rainfall, require Florida citrus trees to be irrigated for optimal production. Citrus tree root systems are exposed to various hydrologic conditions because of soil temperature and water gradients due to tree canopy shading and under-tree microirrigation. The main goal of this study was to evaluate water balance components in a mature citrus orchard grown on central Florida ridge soils with special interest in quantifying rainfall interception by a citrus canopy and its effect on effective rainfall estimation. Soil water content was monitored every 30 min at 10-, 20-, 40-, and 80-cm depths in the root zone both under and outside of citrus tree canopies. Microirrigation, rainfall, and weather data were used to calculate effective rainfall, plant water uptake, and deep drainage. We found that the tree canopy intercepted 35 and 50% of the incoming high (≥5-mm) and low (<5-mm) intensity rainfalls, respectively. Effective rainfall calculated without accounting for the canopy interception effect was over- estimated by about 30 and 5% for the dry and wet periods, respectively. Citrus crop evapo- transpiration was higher under the tree canopy (irrigated area) than outside the tree canopy (unirrigated area) during the dry season because of supplemental irrigation. Abbreviations: CitWatBal, citrus water balance model; ER, effective rainfall; ET c , crop evapotranspiration; ET o , reference evapotranspiration; IRR, irrigation requirement; TR-21, USDA Technical Release 21.