Rainwater use by cotton under subsurface drip and center pivot irrigation

Abstract

To increase the efficiency by which agronomic crops use water from both irrigation and rain during the growing season requires quantifying the proportion of rainfall used by the crop for a rain event. The rainfall pattern in the Texas High Plains (THP) is characterized by isolated thunderstorms of high rates and of short duration, where <1% of the storms produce rain events>50 mm and 80% of total rain events are < than 13 mm. The primary source of irrigation-water in the THP is pumped from the Ogallala Aquifer (OA), which has a different isotopic (δ18O) signature compared to rainfall-captured water. Given this difference, it is feasible to quantify changes in the δ18O signature of the plant water as the plant uptakes the rain- and irrigation-water stored in the soil. To this end, cotton (Gossypium hirsutum L.) was grown and irrigated with subsurface drip and center pivot and under dryland conditions. The irrigation-water was pumped from the OA and rainfall was collected in a rain gauge with mineral oil to prevent evaporation. Additionally, plant and soil samples were collected before and after each rain event. Thereafter, water was extracted from the collected soil and plant samples using cryogenic vacuum distillation and analyzed for δ18O (‰) using a Liquid-Water Isotope Analyzer. The difference in isotope concentrations in the extracts showed a change δ18O (‰) of the cotton petiole water toward that of the rainwater signature of 17% for sub-surface drip, 32% for dryland, and 63% for center pivot irrigation. These results imply that the application of irrigation-water with a sprinkler center pivot results in increased rainwater use in cotton compared to that of sub-surface drip.