Water Requirement and Crop Coefficients of Young Jatropha curcas L. Trees in a Subtropical Humid Environment

Long-term and continuous crop evapotranspiration ($E{T}_c$) and the crop coefficient ($K_c$) of jatropha (Jatropha curcas L.), which are needed to determine irrigation requirements, are lacking. The objective of this study was to measure jatropha $E{T}_c$ and $K_c$ over four growing years. Field experiments were conducted from March 2012 to August 2015, and three different irrigation treatments were evaluated. The $K_c$ values were determined for the irrigated treatments as the ratio between the $E{T}_c$ and grass-reference evapotranspiration ($E{T}_0$) (i.e., $K_c=E{T}_c/E{T}_0$). The $E{T}_c$ was determined using a directly weighing lysimeter, and agrometeorological parameters measured at a weather station were used to estimate the $E{T}_0$. Over the four growing years, the $E{T}_c$ values ranged from 0.7 to $8.3\mathrm{mm}{\mathrm{day}}^{-1}$ for center pivot irrigation, $0.6-7.4\mathrm{mm}{\mathrm{day}}^{-1}$ for drip irrigation, and $0.4-5.4\mathrm{mm}{\mathrm{day}}^{-1}$ for no irrigation (rainfed). In the first growing year, a $K_c$ average of 0.6 for all treatments was found. The highest $K_c$ values were obtained at midphase (from December to February), with values of 0.9 and 0.8 in the second growing year, respectively, for the center pivot and drip treatments. During the same growth phase for the third and fourth growing season, the $K_c$ for both irrigated treatments were averaged at 0.9.