Soil moisture derived using two apparent thermal inertia functions over Canterbury, New Zealand

Abstract

The near-surface soil moisture (SM) is an important property of the soil that can be studied from satellite remote sensing observations over a large spatial domain. This research provides an estimate on the accuracy of SM retrieved from satellite land surface temperature (LST) observations over the Canterbury Plains, New Zealand. The apparent thermal inertia (ATI) method with two approaches (ATI1 and ATI2) was applied to derive the near-surface SM from the moderate resolution imaging spectroradiometer (MODIS) LST product. The in-situ measurements of SM and rainfall data at six sites across the study area were used as reference. The analysis was conducted over two periods, a short period of four months and a longer period of three years. SM simulations by the weather research and forecasting (WRF) model were used in the analysis for the shorter period. Overall, SM based on ATI2 showed a slightly higher correlation with the in-situ measurements (ρ̄= 0.66) than ATI1 (ρ̄= 0.63). The correlation, in general, was higher for the WRF simulations ( ρ̄= 0.81). Both functions performed better during summer compared to winter, but overall, ATI2 showed lower mean errors (ME ≈-15m3 • m-3 volumetric SM) compared to ATI1 (ME ≈-20m3 • m-3) at most of the sites. Additionally, seasonal variations of SM were better detected by ATI2 than ATI1, and the effects of precipitation were detected on more occasions by the ATI2 function. We conclude that ATI2 function can be used to estimate the near-surface SM over a large area from the MODIS LST time series if a few representative reference stations are available. © The Authors.