Print PageMuch of earth’s land area is characterized by shallow soil over weathered bedrock. It is important to develop good estimates of water flux and water storage in these regions, since they often contribute disproportionally to regional water yield. Additionally, they may show quantitatively and qualitatively different responses to climate change compared to soil-based systems that have been studied for much longer. Among the major unknowns for these systems are: 1) What is the contribution of rock-stored water to ET, 2) How do different species partition shallow soil water and deeper rock-stored water, 3) What are the recharge and depletion dynamics of rock-stored water? In the absence of direct data or established methodology to answer these questions, models are a useful tool for inference that may help guide future experimental approaches. Here we introduced a modelling approach that was informed by soil water, sapflow and predawn leaf water potential data captured in the karst area of the Edwards Plateau in central Texas, involving Juniperus ashei, Quercus virginiana var. fusiformis and Prosopis glandulosa. The model consisted of a one-dimensional soil-vegetation-atmosphere water transfer model with parameters fitted to simulate transpiration rates of the three study species.
Results/Conclusions
When water uptake from bedrock was not accounted for in the model, soil moisture depletion did not correspond to observed plant water stress, indicating that bedrock water could be an important water source in this system. A layered model with stratified root patterns performed better than a uniform unlayered model in predicting declines in tree transpiration during summer drought. The modeling exercises also highlighted challenges for modeling ecohydrology of soil/fractured bedrock systems. First, there is uncertainty regarding the nature of water sources below the soil horizon, e.g. do plants extract water from soil lenses in the rock, perched water tables, or the rock matrix itself? Second, we cannot easily resolve whether the rapid onset of water stress in plants during drought indicates low water storage capacity or slow diffusion rates, for example out of the rock matrix. These challenges could potentially be resolved by looking at multiple years incorporating extended wet and dry seasons, and direct assessment of the water dynamics of fractured bedrock. The study of these systems provides opportunities for stronger collaboration between modelers and experimentalists, ecohydrologists and groundwater hydrologists, for the purpose of developing more accurate models of hydrological processes for regions varying in soil depth.
