Variable Source Area Hydrology

Background

The majority of my dissertation research at the Soil and Water Laboratory at Cornell University centered on the advancement of hydrological process understanding controlling the flux of nutrients (e.g. phosphorus) from the landscape to surface water bodies. The goal of this research was to develop an improved mechanistic understanding of hydrologic flow pathways in these landscapes so that biogeochemical hotspots can be better identified using advanced water quality models. This research involved field experiments in a trenched hillslope near Harford NY, water isotope and geochemical tracer studies, application of environmental geophysical methods to characterize hydraulic and physical soils properties, and improvement of a water balance model through implementation of the VSA hydrology concept.

Video above: Animation of shallow water table dynamics and runoff-generation (3D plot) in a 0.5 ha trenched hillslope in upstate NY. The blue bar graph and line graph show hourly precipitation and total discharge data for the period October 23 to November 5 2009. Yellow dots indicate locations of shallow groundwater wells. The red and white lines across the hillslope indicate the flow contributing, saturated slope length for each event, which was calculated based on observed water fluxes atop the flow impeding fragipan soil horizon respectively. The saturated slope length idnicates the maximum distance from which water and nutrients could potentially be transported to the trench during a storm event.
 

Key findings

  • The VSA interpretation of the SCS-CN method accurately predicts VSA extents in small watersheds or plots.
  • If the water table is at a depth of 10 cm below the soil surface, direct runoff in the form of shallow subsurface flow is initiated.
  • Not all storm flow is generated as overland flow due to intersection of the water table with the land surface.
  • The amount of saturation-excess runoff generated during storm events is, to a great extent, controlled by the available soil water storage, which changes daily and seasonally with antecedent moisture conditions.
  • The VSA-interpretation of the SCS-CN method works best if the catchment soil storage, S, is varied with antecedent moisture conditions. Methods such as deriving S from soil moisture data or base flow conditions are recommended.
  • Antecedent moisture conditions control the mobilization of event and pre-event water and predominant flow direction (vertical percolation vs. lateral subsurface flow) during storm events.
  • These hydrologic dynamics have implications for choices of measures to abate nutrient and pollutant loading to streams, for instance indicating possible greater abatement efficiency of variable-width than fixed-width stream buffers.

Af_obs_pred_avg_color.png

 

Figure above: Linear regressions between predicted saturated areas using the VSA interpretation of the SCS-CN method and observed average saturated area extents, derived for different water table depths below the soil surface. Best agreement is achieved if the regression line approaches closely the 1∶1 line. RMSE is the root-mean-square error and E shows the Nash-Sutcliffe coefficient (Nash and Sutcliffe 1970).

 

antec_moisture_CN.png

(a) Relationship between antecedent-moisture-condition-dependend site-specific storage parameter, S, and average depth to water table in hillslope prior to each storm event; (b) relationship between S and base flow observed within 24-h period prior to each storm event. S refers to the potential maximum storage for water available in a watershed as defined in the VSA interpretation of the Soil Conservation Service Curve Number equation. 

Related publications

  • Dahlke HE, Easton, Z.M., Lyon, S.W., Destouni, G., Walter, T., and Steenhuis,T.S. 2012. Dissecting the variable source area concept – Subsurface flow pathways and water mixing processes in a hillslope. Journal of Hydrology, 420421: 125-141.
  • Dahlke HE, Easton, Z.M., Walter, T.M., and T.S. Steenhuis. 2012. Field test of the variable source area interpretation of the Curve Number rainfall-runoff equation. Journal of Irrigation and Drainage Engineering 138(3): 235-244, DOI: 10.1061/ASCE)IR.1943-4774.0000380.
  • Dahlke, H.E., Easton, Z.M., Fuka, D.R., Lyon, S.W. and, T. S. Steenhuis. 2009. Modeling Variable Source Area Dynamics in a CEAP Watershed. Ecohydrology 2, 337-349.
Dr. Helen E. Dahlke - Catchment Hydrology, Experimental Hydrology, Hydrology-Climate Interactions