Biophysical response of young pomegranate trees to surface and sub-surface drip irrigation and deficit irrigation

Due to recurring agricultural water shortages, many farmers are looking for crops such as pomegranate that have both some degree of drought resistance and a higher economic value. To manage limited water effectively, it is necessary to know how pomegranate trees respond to varying levels of applied water and the type of irrigation system used. This study was conducted in experimental orchards located at the University of California Kearney Agricultural Research and Extension (UC KARE) Center and the USDA-ARS San Joaquin Valley Agricultural Sciences Center in Parlier, California. The impact of surface drip (DI) and subsurface drip irrigation (SDI) at the UC KARE orchard and irrigation levels (35, 50, 75, 100% of crop water use) at the USDA-ARS orchard on tree canopy size [fractional canopy ground cover (f(c)), normalized difference vegetation index (NDVI), normalized difference infrared index (NDII)], canopy-to-air temperature difference, and fruit yield was determined during the 2012-2014 growing seasons. There was no difference in tree canopy size between the DI and SDI systems at the UC KARE site; but trees under SDI produced larger fruit in 2013 and higher yield whilst saving about 10% of water application. The difference in water application highlights an advantage of SDI over DI systems to conserve water without creating negative impacts on tree size or yield. At the USDA-ARS site, the 35 and 50% irrigation treatments significantly reduced tree canopy size in 2013 and 2014, compared to the 75 and 100% irrigation treatments. In 2014, trees irrigated with 75% of crop water use produced more yield than trees irrigated at 35% of crop water use. Furthermore, NDVI and NDII were correlated to f(c). The relationships between NDVI to f(c) and f(c) to pomegranate crop coefficient were established. The derived crop coefficient from ground-based remotely sensed data may potentially be used as a tool by farmers and water managers to estimate pomegranate tree water use on a field or regional scale using aerial or satellite imagery.