Aspect differences in vegetation type drive higher evapotranspiration on a pole-facing slope in a California oak savanna

Quantifying the spatiotemporal variability in evapotranspiration is critical to accurately predict vegetation health, groundwater recharge, and streamflow generation. Differences in hillslope aspect, the direction a hillslope faces, result in variable incoming solar radiation and subsequent vegetation water use that influence the timing and magnitude of evapotranspiration. Previous work in forested landscapes has shown that equator-facing slopes have higher evapotranspiration due to more direct solar radiation and higher evaporative demand. However, it remains unclear how differences in plant functional groups (i.e., grasses and trees) influence evapotranspiration and water partitioning between hillslopes with opposing aspects. Here, using field-based measurements of soil water storage and oak tree and grass transpiration, and remotely-sensed evapotranspiration and normalized vegetation difference index, we quantified evapotranspiration and subsurface water storage deficits between a pole-facing and equator-facing hillslopes with contrasting vegetation types within central coastal California. Our results suggest that cooler pole-facing slopes with oak trees have higher evapotranspiration than warmer equator-facing slopes dominated by grasses, which is counter to previous work done in landscapes with singular vegetation types. Our water storage deficit calculations indicate that the pole-facing slope has a higher subsurface storage deficit and a larger seasonal dry down than the equator-facing slope. This aspect difference in subsurface water storage deficits may influence subsequent deep groundwater recharge and streamflow generation. In addition, larger root-zone storage deficits on pole-facing slopes may reduce their ability to serve as hydrologic refugia for oaks during periods of extended drought. This research provides a novel integration of field-based and remotely-sensed estimates of evapotranspiration required to properly quantify hillslope-scale water balances. These findings emphasize the importance of resolving hillslope-scale vegetation structure within Earth system models, especially in landscapes with diverse plant functional groups.

量化蒸散发（evapotranspiration）的时空变异性，对精准预测植被健康状况、地下水补给与径流生成过程具有关键意义。坡向（hillslope aspect）指坡面朝向，其差异会导致入射太阳辐射的分布差异，进而影响植被耗水模式，最终改变蒸散发的发生时序与强度。此前针对森林景观的相关研究表明，朝向赤道的坡面因接收更多直接太阳辐射、蒸发需求更高，其蒸散发水平也相对更高。然而，目前学界仍未明确：植物功能群（plant functional groups，即草本与乔木）的差异，会如何影响不同坡向坡面的蒸散发过程与水分分配格局。本研究依托野外实测的土壤蓄水量、橡树与草本植物的蒸腾速率数据，结合遥感反演得到的蒸散发与归一化植被差指数（normalized vegetation difference index），对加利福尼亚中部海岸区域内两类植被类型迥异的极向坡与赤道向坡的蒸散发及地下蓄水量亏缺进行了量化分析。研究结果显示，相较于以草本为优势植被的温暖赤道向坡，覆盖橡树的凉爽极向坡拥有更高的蒸散发水平——这与单一植被类型景观下的既往研究结论相悖。蓄水量亏缺计算结果表明，极向坡的地下蓄水量亏缺程度更高，季节性干旱消退过程也更为显著。这种由坡向差异导致的地下蓄水量亏缺格局，可能会进一步影响后续的深层地下水补给与径流生成过程。此外，极向坡更大的根区蓄水量亏缺，可能会削弱其在长期干旱期间作为橡树水文避难所的能力。本研究创新性地整合了野外实测与遥感反演的蒸散发估算结果，为精准量化坡面尺度的水量平衡提供了必要的支撑。这些研究结果强调，在地球系统模型中厘清坡面尺度的植被结构尤为重要，尤其针对植物功能群多样的景观区域。