Citrus Water Uptake Dynamics on a Sandy Florida Entisol

Florida citrus trees must be irrigated to reach maximum production due to low soil water-holding capacity. In a highly urbanizing state with limited water resources, improved understanding of soil water uptake dynamics is needed to optimize irrigation volume and timing. The objectives of this study were: (i) estimate mature citrus daily evapotranspiration (ETc) from changes in soil water content (u), (ii) calculate citrus crop coefficients (Kc) fromETc and reference evapotranspiration (ETo), (iii) determine the relationship of soil water stress coefficient (Ks) to u, and (iv) evaluate how ETc was related to root length density. In a 25-mo field study using mature ‘Hamlin’ orange [Citrus sinensis (L.) Osbeck] trees, ETc averaged 1137 mm yr, and estimated Kc ranged between 0.7 and 1.1. Day of year explained more than 88% of the variation in Kc when u was near field capacity. The value of Ks decreased steadily from 1.0 at field capacity (u 5 0.072 cm cm) to approximately 0.5 at 50% available soil water depletion (u5 0.045 cm cm). Roots were concentrated in the top 15 cm of soil under the tree canopy (0.71 to 1.16 cm roots cm soil), where maximum soil water uptake was about 1.3 mm mm root d at field capacity, decreasing quadratically as u decreased. Estimating daily plant water uptake and resulting soil water depletion based on root length density distribution would provide a reasonable basis for a citrus soil water balance model. FLORIDA is one of the fastest growing states in the USA, adding about 700 new residents each day. Concurrently, competition for limited water resources is increasing. Improving our knowledge of soil and plant factors that affect water uptake by Florida citrus trees is essential to optimize irrigation volume and timing so that water can be more efficiently used. Better irrigation scheduling will also reduce negative impacts on ground water quality due to agrichemical leaching through Florida’s highly porous sandy soils. Citrus water requirements vary with climatic conditions and variety (Rogers and Bartholic, 1976; Boman, 1994; Fares and Alva, 1999). Florida citrus ETc typically ranges between 820 and 1280 mm yr (Rogers et al., 1983). In addition to tree uptake, soil water content can be reduced by evaporation from the soil surface and transpiration from noncrop species (Allen et al., 1998). Soils lose their ability to conduct water to the surface as they dry (Hillel, 1998). Likewise, citrus ETc decreases as the fraction of the soil surface receiving full sunlight decreases and the canopy shades an increasingly larger ground area (Castel andBuj, 1992). Conversely, soil water use or apparent ETc increases with increased ground coverage by noncrop species (Smajstrla et al., 1986). The above factors combine to limit ETc for a given crop under specific conditions. Allen et al. (1998) proposed that ETc can be derived from ETo as follows: ETc 5 (ETo)(Kc)(Ks) [1] where ETc is crop evapotranspiration (mm d); ETo is potential evapotranspiration (mm d); Kc is crop coefficient; and Ks is soil water depletion coefficient. Kc is defined as the ratio of ETc to ETo when soil water availability is nonlimiting. In this case, Ks is assumed to be equal to unity. Kc is indicative of climatic and/or developmental effects on ETc compared with ETo. Estimates of Kc for a wide range of citrus tree sizes span from 0.6 in the fall and winter to 1.2 during the summer months (Boman, 1994; Fares and Alva, 1999; Martin et al., 1997; Rogers et al., 1983). As soil water content (u) decreases, soil water potential (f) also decreases, resulting in lower plant soil water uptake and thus lower ETc/ETo ratios. Allen et al. (1998) suggested that for most soils, a value of u less than field capacity (uFC) exists where water uptake is not limited by f. They referred to the range of u above a critical threshold value (ut) as readily available water (RAW), and used it to estimateKs as the ratio of remaining available soil water to soil water that is not readily available (Fig. 1): Ks 5 [TAW 2 (uFC 2 u)]