Land Cover Influences on Watershed Runoff Patterns in Eastern Piedmont of USA

Physiography and land cover determine the hydrologic response of watersheds to climatic events. However, vast differences in climate regimes and variation of landscape attributes among watersheds (including size) have prevented the establishment of general relationships between land cover and runoff patterns across broad scales. This project addressed these difficulties by using 87 watersheds within the same physiographic region (eastern Piedmont, USA), thereby minimizing variation in physiography. Power spectral analysis was used to characterize area-normalized runoff patterns for the 87 watersheds, which were then compared to the attributes of the watersheds. The effect of land cover on runoff patterns was confirmed. Urban-dominated watersheds were flashier and had less hydrologic memory compared with
forest-dominated watersheds, whereas watersheds with high wetland coverage had greater hydrologic memory. We also detected a 10–15% urban threshold above which urban coverage became the dominant control on runoff patterns. When spectral
properties of runoff were compared across stream orders, a threshold after the third order was detected at which watershed processes became dominant over precipitation regime in determining runoff patterns. Finally, we present a matrix that characterizes the hydrologic signatures of rivers based on precipitation versus landscape effects and low-frequency versus high-frequency events. The concepts and methods presented can be generally applied to all river systems to characterize multiscale patterns of watershed runoff.

 

 

 

References

Julian JP, Gardner RH.  2014.  Land cover effects on runoff patterns in eastern Piedmont (USA) watersheds.  Hydrological Processes 28: 1525-1538.

Urbanization Effects on Riverbank Erosion and Channel Geometry

This study identified and assessed the controls on hydraulic erosion of cohesive riverbanks along a 600-m reach of an urban stream in the heart of downtown Aiken, South Carolina, USA.  We examined hydraulic bank erosion by separating bank shear stress into four properties: magnitude, duration, event peak, and variability.  Results showed that the event peak (maximum peak) of excess shear stress best predicted cohesive bank erosion at transects with moderate critical shear stresses (1.93 – 4.08 N/m2), while the variability (all peaks) of excess shear stress best predicted erosion at the transect with low critical shear stress (0.95 N/m2).  These results suggest the increased flow peak intensities from urbanized watersheds increase hydraulic erosion of riverbanks, resulting in deeper and wider channels. The results of this study were combined with results from previous bank erosion studies to produce a conceptual model for estimating bank erosion rates based on their silt-clay content.

References

Julian JP, Torres R.  2006.  Hydraulic erosion of cohesive riverbanks. Geomorphology 76: 193-206.

Effects of Riparian Deforestation on Stream Ecosystems

An emerging issue in stream ecology and ecohydrology is the role of light in fluvial ecosystem dynamics. In this project, we investigated how photosynthetically active radiation (PAR) influences the hydrogeomorphology and biogeochemistry of a 2nd-order temperate stream in central Wisconsin with varying riparian communities; from heavily shaded forest sections to unshaded grass sections. First, in-stream PAR was compared to submerged aquatic vegetation distributions along a 1.2-km reach. We then analyzed the effects of vegetation on water depth, sediment size, sediment volume, organic matter accumulation, and nutrient uptake. Compared to forested sites, deforested sites had three times more benthic PAR, which resulted in a quadrupling of vegetation biomass. This greater biomass at deforested sites increased water depth, sediment accumulation, and the uptake of soluble reactive phosphorous (SRP). Finally, we used the above relations to estimate biogeochemical differences between a completely forested reach and a deforested reach. Compared to a forested reach, the deforested reach accumulated almost twice as much bed sediment and retained more than four times as much SRP. Thus, changes in riparian conditions may create a cascade through which shading drives changes in stream habitat, which in turn drives changes in hydrogeomorphology and biogeochemical cycles.

 

References

  • Julian JP, Seegert SZ, Powers SM, Stanley EH, Doyle MW.  In press, available online.  Light as a first-order control on ecosystem structure in a temperate stream.  Ecohydrology, doi: 10.1002/eco.144.
  • Julian JP, Doyle MW, Stanley EH. 2008.  Empirical modeling of light availability in rivers. Journal of Geophysical Research – Biogeosciences 113: G03022, doi:10.1029/2007JG000601.
  • Riggsbee JA, Manners R, Julian JP, Doyle MW, Muehlbauer J, Sholtes J, Small MJ. Accepted. Influence of aquatic organisms on channel forms and processes. In Treatise in Fluvial Geomorphology. Ed. Wohl E. Elsevier.

Consequences of agricultural land use on light availability and primary production in rivers

While much focus has been directed toward habitat availability and nutrient cycling across the fluvial landscape, light availability has received considerably less attention and had not been assessed quantitatively at the basin-scale despite it being the primary energy source for aquatic ecosystems.  We developed a basin-scale light availability model that couples readily-available broad spatial data with easily-measured synoptic data using a GIS framework and the principles of hydraulic geometry.  We used this model to (i) quantify benthic light availability (Ebed) along a 160-km river in central Wisconsin, USA, (ii) predict gross primary production (GPP) along the same river, and (iii) assess the effects of agricultural land use on Ebed and GPP.  Overall, Ebed decreased in the downstream direction due primarily to increased turbidity, and there was considerable local variation caused by changes in topography, riparian vegetation, and channel orientation. When summed over the entire channel length, present-day, post-agricultural GPP (635 kg C d-1) was ~8 times lower than estimated pre-agricultural GPP (4,992 kg C d-1).  Model simulations revealed that agricultural land use can cause an order of magnitude change in GPP, reduce or increase inter-sectional variability in GPP, and significantly alter broad spatial trends in GPP.

 

 

 

References

  • Julian JP, Stanley EH, Doyle MW.  2008.  Basin-scale consequences of agricultural land use on benthic light availability and primary production along a sixth-order temperate river. Ecosystems 11: 1091-1105.
  • Julian JP, Doyle MW, Powers SM, Stanley EH, Riggsbee JA.  2008.  Optical water quality in rivers. Water Resources Research 44: W10411, doi:10.29/2007WR006457.