Columbia River DSR GFModel IUM IUV Levy stable distribution Monte Carlo SW-GW interaction TOUGH2 Ulan Buh Desert analytical solution assessment bimsoils desert farmland erosion evaporation calculation evolving-scale log-conductivity field measurements first-order analytical approach flow modeling fractional derivative fractured aquifers groundwater ERT groundwater flow groundwater flow model heterogeneity hydraulic conductivity hydrologic exchange infiltration irrigation