aeolian processes aquifer center pivot irrigation colorado colorado river conservation of mass dust bowl erosion farmer geographic coordinate system great plains groundwater hydraulic conductivity hydrogeology ice age irrigation kansas livestock maize miocene nebraska nevada new mexico oklahoma platte river pliocene porosity pump rocky mountains south dakota spider-man steppe sunflower surface water texas texas panhandle united states united states geological survey vadose zone water cycle water table water well wheat world war ii wyoming