Simulated soil water distribution patterns and water use of alfalfa under differing subsurface drip irrigation depths

Authors: ELTARABILY, MOHAMED GALAL - University Of California, Davis; MOHAMED, ABDELMONEIM - Kearney Agricultural Center; Begna, Sultan; Wang, Dong; PUTNAM, DANIEL - University Of California, Davis; SCUDIERO, ELIA - University Of California, Riverside; BALI, KHALED - Kearney Agricultural Center

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2024
Publication Date: 1/24/2024
Citation: Eltarabily, M., Mohamed, A.Z., Begna, S.H., Wang, D., Putnam, D.H., Scudiero, E., Bali, K.M. 2024. Simulated soil water distribution patterns and water use of alfalfa under differing subsurface drip irrigation depths. Agricultural Water Management. 293. Article 108693. https://doi.org/10.1016/j.agwat.2024.108693.
DOI: https://doi.org/10.1016/j.agwat.2024.108693

Interpretive Summary: Alfalfa is one of the main perennial forage crops in the U.S., vital for livestock and forage industries and environmental benefits. Most of the production is in the western U.S. with the bulk of this production in California. Production in the western U.S. is under arid/semi-arid growing conditions heavily dependent on irrigation water. Flood irrigation system is the main method, but it is the lowest efficient of all known irrigation systems. With growers facing challenges with water shortages, they are looking for alternative efficient irrigation system such as sub-surface drip irrigation (SDI) system but information on SDI in alfalfa is limited especially in drip tapes spacing and depths placement and their effect on topsoil profile waiting patterns and alfalfa crop-water productivity. SDI can minimize deep percolation and ensure a dry topsoil surface for efficient harvesting of alfalfa. A field research was conducted at University of California, Desert Research and Extension Center, Imperial Valley, California. Three different depths of drip tapes placed at 15, 30, and 45 cm below soil surface were compared. Crop water requirement was calculated using Tule sensors-based system while soil matric potentials along 120 cm depth (15cm apart) were monitored using watermark soil moisture sensors. Soil moisture content from twelve cuts in 2020 and 2021 (April 2020 thru July 2021) was simulated using HYDRUS-2D. Results showed that there was no significant difference between root water uptake (RWU) among drip depths. RWU accounted for 196, 197, and 182 cm at drip line depths of 15, 30, and 45 cm, respectively. A 10 % reduction in topsoil moisture content occurred, specifically near the drip line, for drip lines at 30 cm depth compared with 15 cm while a 20 % reduction of topsoil moisture content occurred at 45 cm depth compared with 15cm depth. Drip lines at 30 cm depth seemed to be optimal for RWU; however, drip lines at 45cm depth may allow growers to practice additional irrigation event (without increasing the topsoil moisture content to be similar as 15 cm depth) closer to harvest date, potentially resulting in higher yield and water use efficiency. This can be used by growers as an irrigation management strategy to improve alfalfa crop-water productivity in Imperial Valley and other regions of California.

Technical Abstract: Alfalfa is one of the most cultivated forage crops in the US, where nine percent is grown in California. Maximizing yield while obtaining high water use efficiency becomes challenging. Subsurface drip irrigation (SDI) can minimize deep percolation and ensure a dry topsoil surface for efficient harvesting of alfalfa. The objective of this study was to compare different depths of drip lines for achieving sustainable management practices of SDI. An alfalfa research experiment was conducted at University of California, Desert Research and Extension Center, Imperial Valley, California to assess three different depths of drip tape placed at 15, 30, and 45 cm below soil surface. Crop water requirements were calculated by Tule sensors (Tule Technologies Inc.) installed in the field, while soil matric potentials along 120 cm depth (15cm, apart) were monitored using watermark soil moisture sensors. A previously acquired soil moisture retention curve for the same soil type was used for soil moisture calculations. Soil moisture from twelve cuts in 2020 and 2021 (from 6th April 2020 to 23rd July 2021) was simulated using HYDRUS-2D. Initial soil moisture content was assigned from watermark readings on 6th April 2020, while actual soil moisture distribution across depth was compared with watermark readings for hydraulic conductivity calibration. Results showed no significant difference between root water uptake (RWU) among drip depths. RWU accounts for 195.59, 197.09, and 182.18 cm at drip line depths of 15, 30, and 45 cm, respectively. Applied irrigation water during the study period accounted for 193.47 cm while rainfall was 4.12 cm. A 10% reduction in topsoil moisture was obtained, specifically near the drip line, for drip lines at 30 cm depth compared to 15 cm, while a 20% reduction of topsoil moisture was found at 45cm depth compared to 15cm depth. Drip lines at 30 cm depth are optimal for RWU; however, drip lines at 45cm depth can give a chance to apply additional irrigation events (without increasing the topsoil moisture similar to 15 cm depth SDI) closer to harvest date, possibly resulting in higher yield. A detailed feasibility study for the installation cost of the irrigation system at deeper depths than the shallow relative to the return value from the yield increment is recommended as a further analysis.