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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 (ET c ) from changes in soil water content (u), (ii) calculate citrus crop coefficients (K c ) from ET c and reference evapotranspiration (ET o ), (iii) determine the relationship of soil water stress coefficient (K s ) to u, and (iv) evaluate how ET c was related to root length density. In a 25-mo field study using mature 'Hamlin' orange [Citrus sinensis (L.) Osbeck] trees, ET c averaged 1137 mm yr 21, and estimated K c ranged between 0.7 and 1.1. Day of year explained more than 88% of the variation in K c when u was near field capacity. The value of K s decreased steadily from 1.0 at field capacity (u 5 0.072 cm 3 cm 23 ) to approximately 0.5 at 50% available soil water depletion (u 5 0.045 cm 3 cm 23 ). Roots were concentrated in the top 15 cm of soil under the tree canopy (0.71 to 1.16 cm roots cm 23 soil), where maximum soil water uptake was about 1.3 mm 3 mm root 21 d 21 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.

Fine sand soils important to Florida agriculture have volumetric soil water content values (θv) of <0.10 cm3 cm−3 after drainage due to gravity has ceased. Small changes in θv in the range of 0.02 to 0.08 cm3 cm−3 can greatly affect plant available water and, therefore, good calibration of soil water content sensors is necessary. The EnviroSCAN (Sentek Pty. Ltd., South Australia) is a multiple sensor capacitance probe capable of continuous measurement of soil water content by volume (θv). Many fine sand soils in Florida have plant available θv values of ≤0.08 cm3 cm−3 The manufacturer's calibration curve has very few data points <0.10 cm3 cm−3 θv and no data in the 0.02 to 0.04 cm3 cm−3 θv range. Because of the lack of data in this range, a calibration curve from 0.02 to 0.08 cm3 cm−3 θv was developed for Candler fine sand (hyperthermic, uncoated Typic Quartzipsamments), Apopka fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudults), and Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Alaquods) in two locations in Florida. Since calibration curves for the three soils did not differ significantly, data from the three soils were combined. An exponential calibration curve was developed This equation provides substantially different estimates of water content in the 0.02 to 0.08 range than values obtained from the manufacturer's calibration. This improved calibration extends the useful range of the EnviroSCAN to include an important group of soils with very low water holding capacity.