I've used simple models to understand the relationship between soil moisture and evapotranspiration in this paper. I have also used these same simple models to understand the origin of soil moisture trends in satellite and in-situ observations. A final manuscript currently in prep concerns soil moisture and agriculture. A few more questions are:
  • Does the soil moisture state influence precipitation? If so, how?
  • How much memory does soil moisture provide to the land surface?
  • What's the relationship between relative humidity and soil moisture?
  • Can we use the relationship between temperature and soil moisture to understand plant mortality?