Monitoring Network for Riparian Evapotranspiration using the Sub-Surface Water Balance
Riparian evapotranspiration (RET) is an important component of basin-wide evapotranspiration (ET), especially in subhumid to semi-arid regions, with significant impact on water management and conservation. A common method of measuring ET is using the eddy correlation technique. However, since most riparian zones are narrow eddy correlation techniques are not applicable because of limited fetch distance. Another typical method for measuring ET is using the Surface-Subsurface Water Balance (SSWB).  The accuracy of this method is not well understood due to factors such as heterogeneity of soil properties, plant diversity and their root distribution within the riparian zone.  To gain a better understanding of RET estimation using the SSWB method a 100 meter long by 40 meter wide riparian zone  along Rock Creek  in the Whitewater Basin in central Kansas was selected. Main features of this study site include a thick vadose zone with fine-grained soils underlain by a phreatic zone of coarse gravel embedded in clay which seems to act as an aquitard, all of which overlies a karst bedrock.  The study area was instrumented with 6 soil moisture profiles with each profile containing 4-5 sensors distributed down to a depth of about 1.8 meters.  In addition to the soil moisture profiles, ground water monitoring wells were installed near each of the profiles along with one deep bedrock well, 4 shallow wells and one in the stream. To measure the stream flow two stream gage sensors were installed; one upstream and the other at the downstream end. Meteorological data was also measured by installing a weather station to the south of the riparian zone. 
Initial results from both the soil moisture and groundwater well network indicate that there is a great degree of heterogeneity within the subsurface system. From the soil moisture data it can be observed that there is a great variation of moisture within each of the profiles.  The ground water wells also show that there is discontinuity between the shallow and deeper wells, where the deep wells indicate a higher head than the shallow wells.  Due to pumping from a nearby well, fluctuations are evident in the deep wells but not in shallow wells or the well in the stream. 
To further understand the heterogeneity of the soil at different depths, laboratory soil experiments are being conducted. These experiments include determining the Soil Water Retention Curve (SWRC) and its hydraulic conductivity by using a laboratory apparatus known as the HYPROP system. A permeameter is also used to validate the hydraulic conductivity of the soil determined by the HYPROP. Other experiments include determining the soil-specific moisture calibration function by taking several soil moisture measurements at different degrees of saturation using a soil calibration box and taking readings with the same type of sensors used in the field. In addition to the monitoring network and laboratory soil analysis a numerical model is being developed to illustrate the controls on the water balance.  The overall goal of this research is to effectively measure the water fluxes within a single riparian link, which then can be scaled to a basin wide stream network and longer time scales.