Water potential dynamics in a precipitation pulse experiment: comparing direct and remote sensing metrics at the leaf scale

Plant water potential is a dynamic and fundamental driver of carbon and water fluxes, yet observations over time remain sparse. In a Sonoran Desert grassland, we utilized a precipitation manipulation experiment to a) evaluate the temporal trajectory of plant water potential following different precipitation pulse sizes and b) relate plant water potential to remote sensing proxies at the leaf scale. Beginning in 2020, natural summer rainfall was excluded and replaced with consistent irrigation divided among three watering treatments, which received identical seasonal totals delivered in return intervals of 3.5, 7, and 21 days (P3.5, P7, and P21, respectively), with correspondingly varied event magnitudes, between July and September. In 2023, we measured predawn and midday leaf water potential (Ψ_PD and Ψ_MD) as well as leaf-level hyperspectral on Digitaria californica, a native perennial bunchgrass, characterizing pulse events on Aug 14 (all treatments) and Aug 21 (second pulses for P3.5 & P7 only). Two spectra per leaf were measured, corrected for known breakpoints, and averaged prior to calculating NDVI, NDWI, and PRI.

Prior to the Aug 14 pulse irrigation, Ψ_PD was above -2 MPa in P7 while P3.5 and P21 both exhibited Ψ_PD around -2.5 MPa. While Ψ_PD peaked on day 1 following irrigation in all treatments, the amount of time spent in the well-watered range differed greatly. Ψ_PD dropped after day 1 in P3.5, after day 2 in P7, and after day 12 in P21, consistent with the varying pulse magnitudes. Uniquely in P21, pulse irrigation increased soil water content at 25 cm, indicating the availability of deeper soil moisture to D. californica with fewer/larger precipitation events. When comparing the replicated pulse events in the frequent/smaller treatments, the water potential response to Aug 14 and Aug 21 pulses differed greatly in P3.5 but not in P7. While soil water contents were similar across pulses, Ψ_PD in P3.5 started above -1 MPa during the Aug 21 pulse and did not exhibit a peaked response, coinciding with lower cumulative VPD. Reduced atmospheric demand may significantly moderate water potential responses to small precipitation events. Finally, across treatments and time-of-day, leaf water potential was most closely correlated with greenness indices of NDVI and PRI (R² = 0.166 and 0.183, respectively), while only loosely correlated with the water content index NDWI (R² = 0.018). Next, we intend to develop our own indices that can better capture the temporal response of leaf water potential to precipitation dynamics.