Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2015
Publication Date: 8/1/2015
Citation: Goebel, T.S., Lascano, R.J., Payton, P.R., Mahan, J.R. 2015. Rainwater use by irrigated cotton measured with stable isotopes of water. Agricultural Water Management. 158:17-25.
Interpretive Summary: Cotton is the major field crop in the Texas High Plains and due to the decline of the irrigation water from the Ogallala aquifer a shift towards more dryland production is anticipated. Currently, the majority of the irrigation wells in the Texas High Plains lack the capacity to produce the daily water requirement of the cotton crop, especially during the critical stages of crop growth. This type of irrigation is called deficit-irrigation, whereby only a percentage of the daily water requirement of the crop is applied. Emphasis is now placed on water conservation and making better use of the rain that crop receives during the growing season. Of interest is to determine under field conditions, how much water a cotton crop transpire that originates from rainwater and from irrigation water applied with a sprinkler and a sub-surface irrigation. To determine this we measured the stable isotope of water (d18O) in cotton leaves after a one-inch rain, every two hours for two days, on cotton that was irrigated with a sprinkler system and a sub-surface drip. Our results showed that indeed stable isotopes are a technique that can be used for this purpose and under field conditions. Our results are the first to show these measurements under field conditions and our results will be used to better manage the remaining irrigation water and to make better use of rainfall.
Technical Abstract: As the world’s population increases, more pressure will be applied to existing groundwater resources to produce the necessary food and fiber to feed and clothe the population. While some groundwater systems used for agriculture are recharged, others such as the Southern Ogallala Aquifer are likely not. As aquifer depletion continues, efficient management of the total water budget, inputs and outputs, of agricultural crops is important. Of interest is the use of rainwater by irrigated crops during the course of the growing season, as rain could account for an important amount of the water input in the semi-arid climate of the Texas High Plains. Our objective was to measure rainwater use on sprinkler and sub-surface drip irrigated cotton (Gossypium hirsutum, L.) under field conditions. Stable isotopes of water were used to quantify rainwater uptake by cotton because the irrigation water from the Ogallala Aquifer has a stable isotopic signature that can either be enriched or depleted when compared to the isotopic signature of water from any rain event. Cotton petioles were sampled before a 25-mm rain, and after every two hours for two days. The water in the petioles was extracted using cryogenic vacuum distillation and was analyzed for its’ isotopic signature. The results showed a shift of 29 % from -7 (‰) to - 5 d18O (‰), which is similar to the isotopic signature of the rainwater (-4.2 d18O (‰)). These results suggest that it is possible to use water stable isotopes to discriminate between rainwater and irrigation transpired by cotton under field conditions.