Estimating the soil water retention curve by the HYPROP-WP4C system, HYPROP-based PC NN-PTF and inverse modeling using HYDRUS-1D

Due to the difficulty of direct field measurement, soil hydraulic properties are often obtained in laboratory settings using small undisturbed soil samples or estimated indirectly through pedotransfer functions (PTFs). The pseudo-continuous pedotransfer function (PC NN-PTF) is a neural network-based approach for estimating soil hydraulic properties. The main objective of this study was to use field soil moisture and tension data to assess soil water retention curves obtained from the HYPROP-WP4C system and the HYPROP-based PC NN-PTF. The in-situ soil water retention data were simultaneously acquired using Acclima TDT and MeterGroup MPS-6 sensors (every 30 min, May to September 2020) from 11 hybrid bermudagrass plots under different irrigation treatments in Riverside, California. We utilized extended evaporation and dewpoint methods using HYPROP-WP4C (Meter Group Inc., USA) devices to obtain lab-measured SWRCs. In addition, SWRCs were estimated from Rosetta (Schaap et al., 2001) and by inverse modeling in HYDRUS-1D utilizing in-situ moisture retention data. Although the hysteresis impacted field data, overall, there was a good agreement between the in-situ and lab water retention data for most samples, especially within the pF range of 2 -3.5. The PC NN-PTF outperformed Rosetta in estimating laboratory (RMSE=0.034 cm 3 cm-3 vs 0.063 cm 3 cm-3 ) and in-situ soil moisture data (RMSE=0.048 cm 3 cm-3 vs 0.082 cm 3 cm-3 ). Inverse modeling of in-situ data also performed well in estimating the SWRC (RMSE=0.043 cm 3 cm-3 ); however, further attention is required in dry and saturated soil conditions. We developed a simple, free, and easy-to-access tool called PC-PTF for estimating the SWRC using the PC NN-PTF model evaluated in this study. The PC-PTF can be accessed from the Verdi Water Management Group website: htt p://www.ucrwater.com/software-and-tools.html.