麻豆影院

Zeliff, Morgan听¹听;听Williams, Mark听²听;听Cowie, Rory听³听;听Knowles, John听⁴听;听Burns, Sean听⁵

¹听University of Colorado
²听University of Colorado
³听University of Colorado
⁴听University of Colorado
⁵听University of Colorado

Here we evaluate how evapotranspiration (ET) and deep percolation (DP) impact the precipitation-runoff response, aquifer recharge, and linked nutrient-cycling at the 536 ha sub-alpine Como Creek drainage in the Colorado Front Range. ET is measured continuously using eddy covariance, soil moisture (SM) is measured using 2-m vertical sensor arrays, groundwater (GW) by a series of piezometers, and precipitation (P) is measured daily along with snow-water equivalent (SWE). Wet precipitation chemistry from a 25 year record collected by the National Atmospheric Deposition Program and hydrochemistry from surface water, groundwater and snowpack are analyzed from the Como Creek catchment as well as an adjacent, higher-elevation catchment.

From 2004 to 2009, annual P averaged 813 mm and ET averaged 590 mm, with ET thus representing 72.5% of annual P. Using multiple linear regression analysis, discharge (Q) was found to be modeled reasonably well with the independent variables of ET (p < 0.01), P (p < 0.01), and SM (p < 0.01). The final linear model had a reasonable fit (r2=0.57) indicating ET, P and SM to be good predictors of Q, with ET and SM having positive coefficients, and somewhat surprisingly, P having a negative coefficient. We found ET to be positively correlated with summer P (r2=0.45), but not well correlated with annual or winter P. Our vertical soil moisture arrays show that summer precipitation over 5 years never penetrated more than 50 cm in depth. Thus, during the summer, water flux in the root zone becomes decoupled from the ground water system and subsequent precipitation does little to contribute to streamflow for the current year, but serves to offset ET, which may explain the decrease in Q with increasing P. The newly installed piezometers (12, at depths ranging from 5 to 30 m) provide evidence that this portion of the basin is largely a losing reach during snowmelt, with GW in the piezometers increasing 5-7 m. After peak snowmelt however, the reach starts gaining again with piezometer levels dropping. Time series plots reveal a strong relationship between SWE and Q with larger SWE often resulting in larger Q. Thus, surface-groundwater interactions are tightly coupled during snowmelt, with snowmelt first replenishing the subsurface water deficit before contributing to discharge. Over the 1.5 years of monitoring, the deepest two piezometers (18 and 27 m) were not showing any significant water level declines, suggesting that water loss to DP is a potential important component of the water balance in the Como Creek catchment. Groundwater and Como Creek nutrient levels contrast with the surface water and snowpack hydrochemisty from higher-elevations where there has been a trend of increasing inorganic nitrogen (Williams & Tonnessen 2000). The reason may be that snowmelt first infiltrates into the subsurface, where ammonium and nitrate are assimilated in the densely forested Como Creek catchment.

Williams, MW and Tonnessen,KA, 2000, Critical loads for inorganic nitrogen deposition in the Colorado Front Range, USA: Ecological Applications, v.10, p. 1648-1665.