Prediction of the evapotranspiration rate of jujube using lysimeters for drip irrigation

Measuring field evapotranspiration can provide important information needed for estimating soil moisture and crop water stress and premature drying out, and such information is essential for irrigation formulation. Evapotranspiration can be measured using large lysimeters that have the advantage of confining soil boundaries, flexible measuring intervals, and high precision. Most of studies of evapotranspiration have been done for annual crops such as wheat and maize, and the present study was conducted to measure evapotranspiration of jujubes. Jujube trees of four years were transplanted into lysimeters and evapotranspiration was measured at 30-min intervals for complete growth season. The correlation between evapotranspiration rate and leaf area index, meteorological factors, and surface soil moisture content was analyzed based on water balance and the PM formula. The daily evapotranspiration of jujube was unimodal, taking place mainly in the daytime; the contribution of was small and stable. Evapotranspiration peaked at flowering stage, reaching 4.42 mm·d^-1, and then declined gradually. The total evapotranspiration during growth season was 640.83 mm, a large proportion of which occurred during flowering and fruit development stages that accounted for 38.61% and 32.72%, respectively. The observation suggested that there is a need for flowering and fruit stages to be emphasized in irrigation of jujube trees. Hourly and daily evapotranspiration rates of jujube were different in their affecting factors. The wind speed (V) affected hourly evapotranspiration only. The most sensitive factor for evapotranspiration was canopy net radiation (R_n), followed by air temperature (T), wind speed (V), leaf area index (LAI), and surface soil moisture content (W), as summarized in the following empirical equations for hourly and daily evapotranspiration, respectively:ET₁(h)=0.153 + 0.004T+ 0.012V+0.176R_n+0.002W+ 0.067LAI, and ET₂(d)=-3.325 + 0.081T+0.163R_n + 0.069W+2.089LAI. Because canopy net radiation had the largest and most significant impact (the partial correlation coefficient was 0.562^** and 0.468^** for the hourly and daily equation, respectively), the regression was simplified as ET₁(h)=0.232 6R_n + 0.018, R²=0.719 6, and ET₂(d)=0.321 2R_n-0.141 8, R²=0.719 6. These equations were tested to be accurate and could be used to estimate the evapotranspiration rate of jujube for developing drip irrigation in arid areas when input data were complete or partially complete.