SOIL WATER FLUCTUATIONS IN A LOAMY SAND UNDER IRRIGATED POTATO.

Diagnostic indicators were developed to evaluate water storage, infiltration, drainage, and uptake distribution patterns over a potato (Solanum tuberosum L.) hill to improve efficiency in drip and sprinkler irrigation. Soil water content time series were measured in the hills and in the furrow between hills using time domain reflectometry. The drainage indicator was significantly (P = 0.05) higher under sprinkler than drip beneath the furrow and at the toe location. Storage was low at the center under sprinkler irrigation (0.05 m3 m-3) where irrigation water did not infiltrate. Water storage was low at the shoulder location under both irrigation systems (significantly greater for sprinkler than drip) where high uptake rates were indicated. A diurnal/sinusoidal model of water uptake was fitted to a composite of the daily cycle of low-amplitude fluctuations with acceptable results (R2 = 0.39-0.72) for both systems at the shoulder and toe locations and at the center under drip. Uptake was virtually undetectable (R2 = 0.12-0.18) at the base and furrow locations. Deviations from the sinusoidal model were better accounted for by a polynomial model (R2 = 0.29-0.83) and seemed to indicate irrigation-induced flow and nonsinusoidal uptake. Electronic noise also contributed to the variability. The indicator of peak uptake time shifted from about 3 p.m. to 8 p.m., and uptake decreased as the depth from the top of the hill increased. Management practices targeted at wetting the hill center under the sprinkler would likely improve water use efficiency under these conditions.