A Critical Assessment of Geological Weighing Lysimeters: Part 1—Monitoring Field Scale Soil Moisture Storage

ABSTRACT Soil moisture plays a critical role in the exchange and partitioning of water and energy fluxes at the land surface. Representative measurements of field scale (10 4 –10 6 m 2 ) soil moisture are needed to characterise hydrological processes, and to calibrate and constrain hydrology, weather and climate predictions. However, measuring soil moisture proves difficult as it varies spatially, as a function of heterogeneity of the land‐surface, and temporally, as a function of dynamic drivers of precipitation, snowmelt, and evapotranspiration. Conventional soil moisture measurement techniques provide point scale (< 1 m 2 ) estimates, while remote sensing applications provide large‐scale estimates (> 10 6 m 2 ). While both spatial scales are useful for certain applications, these methods must be upscaled or downscaled to estimate field‐scale soil moisture. Geological weighing lysimeters (GWL) provide a method to measure total water storage at field scale. This instrument utilises the relationship between hydraulic head and mechanical loading to quantify change in water mass on the lands surface. In this study, we examine a GWL's ability to estimate total water storage and to partition soil moisture storage using complementary observations of snow storage and shallow groundwater storage. We found the GWL record provided credible estimates of total water storage dynamics at our site. The GWL derived soil moisture captured the observed seasonal dynamics at the site. We interpreted the differences in GWL and point scale soil moisture as being due to (i) the dielectric probes inability to sense soil ice content in the winter; and (ii) sub‐field scale heterogeneity in the fluxes (evapotranspiration and drainage) in the summer and fall. We show that GWL storage estimates are credible, and when combined with supplementary storage observations, they can provide accurate estimates of soil moisture storage and valuable insights into the hydrological processes occurring at the site.