On the suitability of non-dormant alfalfa to tolerate off-season groundwater recharge during winter and spring periods

Winter groundwater recharge, which involves flooding farmland with excess surface water during the dormant season to replenish underlying aquifers, is a promising water conservation strategy, and alfalfa represents a particularly promising crop for this practice. In this study, we investigated yield and forage quality effects of intentional winter or spring flooding of non-dormant alfalfa (Medicago sativa L.) for groundwater recharge. A replicated randomized complete block design study was implemented at the University of California Kearney Research and Extension Center (KARE) in Parlier, CA testing three treatments (3 days of flooding followed by 4 days without flooding, 4 days of flooding followed by 10 days without flooding, and control) for a duration of 6 weeks in winter 2019 and spring 2020. A total of 1.6 and 3.6 m³ m⁻² (2019) and 1.3 and 2.2 m³ m⁻² (2020) were recharged in the 4 on 10 off and 3 on 4 off flood treatments at a recharge efficiency of > 88 % (81 % – 217 % of annual irrigation demand), respectively. The 2019 (Feb – Mar) experiment did not show significant differences in alfalfa yield in the first and second cutting after flooding but resulted in slight but non-significant decline in forage quality (e.g. crude protein; fair hay quality). The 2020 flooding experiment, conducted much later in the year (April – May), resulted in no significant differences in alfalfa yield but poor feed quality (utility grade), partly due to delays in harvesting. Together these results indicate that while caution is appropriate to prevent oxygen deprivation and impacts on alfalfa yield and quality, winter recharge in alfalfa fields in highly permeable soils appears to be a viable practice to conserve large quantities of surface water when available in excess.