Geomorphology as a first order control on the connectivity of riparian ecohydrology

Geomorphic controls on ecohydrological connections, while implicit in processes such as the derivation of transpired water from alluvial aquifers, have not been systematically investigated. In this article we test the hypothesis that geomorphic setting is an important control on groundwater-sourced riparian evapotranspiration (ET) through a statistical investigation of 30 years of daily flow data, monthly precipitation data, and semimonthly satellite images at fourteen 1-km-long river reaches representing a range of geomorphic settings in the semiarid landscape of New Mexico, USA. We focus on correlations between vegetation greenness (as a surrogate for vigor and transpiration rates), interannual variability in flow and rainfall characteristics, and valley metrics such as width, slope, and concavity. Sites were considered individually and in groups based on classification as perennial or intermittent. Principal component analysis (PCA) was used to distill related metrics into significant, uncorrelated, interpretable, underlying variables. Components were interpreted in the same manner as a factor analysis and were found to include overall magnitude of greenness, within-summer variability of greenness, overall wetness, rain vs. flow dominance, snowmelt vs. monsoon dominance, and mountain vs. basin geomorphic setting, among others. To broadly test the hypothesis that geomorphology affects the relationship between wetness and greenness, best subset regressions were constructed for the full range of sites and groups using all wetness and geomorphic components as potential predictor variables and greenness metrics as response variables. From our analysis we draw two main conclusions: (1) including geomorphic parameters provides new information that is nonredundant with precipitation and flow parameters in predicting vegetation extent and vigor. The parameters, especially curvature, may indicate causal processes, such as increased soil moisture availability on concave surfaces with convergent flow paths, or they may provide correlative information on the groundwater setting (gaining vs. losing). (2) When considering groups of sites, the dominance of precipitation over stream flow in delivering moisture was a predictor of high greenness; but this was not the case for any individual sites. This indicates that while greener communities developed in sites with greater average precipitation, at any single site the vegetation was able to adapt to either local precipitation or streamflow delivery of moisture in a given year and only the magnitude of the moisture delivered affected vegetation vigor. These findings emphasize the versatility of riparian vegetation to utilize soil moisture in convergent geomorphic settings or wet time periods, moisture that may otherwise tend to evaporate.