Algorithms to Process Weighing Lysimeter Data

Highlights Weighing lysimeters can accurately determine water balance components. Algorithms were developed to automate processing of lysimeter water storage data. The algorithms were tested by comparing results to independent spreadsheet calculations. Discrepancies between spreadsheet and algorithm evapotranspiration were small. Longitudinal graphs were used to detect uncertain or spurious evapotranspiration. ABSTRACT. A set of algorithms written in MATLAB was developed and tested for weighing lysimeters to process 5-minute raw relative storage and event flag data to determine water balance components. Water balance components included non-rainfall water (NRW; i.e., dew or frost), precipitation (P), irrigation (I), drainage (D), and evapotranspiration (ET). The MATLAB program produced output files and graphs for quality assessment of ET results. The program reduced processing time required compared with a spreadsheet that had been used for the same purpose. The program was tested by comparing daily and cumulative NRW, P, I, and ET calculated using the algorithms to those calculated with the spreadsheet. Comparisons included three full calendar years and four lysimeters located at the USDA Agricultural Research Service in Bushland, Texas, USA. Two years (2016 and 2018) included maize seasons, and one year (2019) included a soybean season. P and I amounts calculated from mass data were identical for the spreadsheet and program. The program reproduced the general trend of cumulative NRW, with most NRW occurring in the winter and early spring and very little during the summer at the study location for both MESA and SDI lysimeters. Annual NRW totals calculated using the spreadsheet and the algorithms were 42 to 78 mm. The spreadsheet vs. algorithm root mean squared error (RMSE) for daily NRW were 0.048 to 0.083 mm (~35% to 57% of spreadsheet mean NRW). The algorithms tended to overestimate NRW compared with spreadsheet calculations in 2016 (mean bias error, MBE was 0.033 to 0.050 mm) and underestimate in 2018 and 2019 (MBE was -0.015 to -0.030 mm). Annual ET totals calculated by the spreadsheet and the program were 952 to 1083 mm in 2016; 880 to 1016 mm in 2018, and 1058 to 1111 mm in 2019. Spreadsheet vs. program RMSE for daily ET were 0.11 to 0.25 mm (4.3% to 7.9% of spreadsheet mean ET) and MBE were -0.070 to 0.074 mm. The program was able to identify uncertain or spurious ET results; examples included mis-flagged events, prolonged precipitation over three days, and a leaking drainage line from a lysimeter. Keywords: Dew/frost, Drainage, Evapotranspiration, Irrigation, Maize, Precipitation, Soybean, Sprinkler, Subsurface drip, Water balance.