Measurement of Seasonal Variation in Vertical Root Uptake Distribution Using High Resolution Soil Moisture Data

Understanding root water uptake in response to variability in ambient soil moisture is the most important link between soil science and ecology. Vast knowledge exists in the literature concerning soil moisture dynamics in response to various hydrological processes. Although the importance of roots in affecting soil moisture dynamics is recognized and many models exist, relatively few studies have been directed at measuring the uptake distribution with depth and moisture content. Root water uptake models are mostly empirical and tend to ignore plant physiology. The current study aims at understanding how the variability of soil moisture and root density distribution affects root water uptake. To fulfill the above objective, soil moisture probes were installed along a flow path encompassing two different land covers at a study site located in West Central Florida. Soil moisture data were collected at high vertical and temporal resolution below the land surface, to levels below the seasonal low water table depth. Water table monitoring wells were installed adjacent to soil moisture probes to record concurrent readings of depth to water table. Extensive root density profiling for two land covers, comprising of un-grazed bahia grass and a wetland forest, was performed to determine the corresponding root distribution. Analysis of variation in the soil moisture along with the root distribution shows that under wet or normal conditions the root water uptake is directly proportional to the root density distribution. For dry conditions however, the uptake rate and distribution is proportional to moisture profile with reduction in evapotranspiration occurring below variable thresholds levels. The study suggests that modeling of the evapotranspiration stress in plant communities can be improved by quantifying the vertical distribution of both root mass and water content. The results of the study will be useful for refining model conceptualizations of root water uptake, extinction depth, and ultimately, for evaluation of stress level in plants under drying conditions.