The Basics of Hillslope Hydrology with Examples from Tracer Studies in the Boulder Creek Critical Zone Observatory Eve-Lyn Hinckley Intro to Hydrology 3 Dec 2010

Historical Developments in Hillslope Hydrology Or: How We’ve Explained in Various Ways that Water Runs Downhill Freeze, 1974

Hydrologic pathways through the landscape Hydrologic flow paths are the connection between land and water systems, affecting the distribution of reactive elements, and modifying the physical, chemical, and biological factors that control the fates and forms of reactive elements. Much work has been done to characterize the different hydrological flow paths within the terrestrial system, including… Inspired by Dunne, 1980

Hydrological flow paths affect stream discharge patterns. Cowie, 2010 Hydrological flow paths affect stream discharge patterns.

Infiltration excess (“Horton”) overland flow Robert E. Horton circa 1945 Photo: K. Caylon

Saturation overland (“Dunne”) flow Courtesy: The Comet Program Infiltration Rainfall Rate Overland flow Time Tom Dunne in a Happy Valley trench circa 1970s

Where and when do we observe these different mechanisms of overland flow?

Event-based study of wetting and drying within catchments. Research by Wilson and Dietrich in Northern California.

(physics-based, distributed response) How many holes can you stick in the ground? We need models, and there are many different approaches… TOPMODEL (topography-driven) InHM (physics-based, distributed response) After Loague, 2006

Hot off the press Stable isotope studies show that trees access and transpire different water than appears in streams! The ecohydrologists highlight the role of “the green stuff” (a.k.a. trees).

Add humans: climate and land use change + manipulation of water resources around the world change hillslope hydrology

How are we changing hillslope processes? Climate change: altering precipitation amounts/timing and temperature patterns Agriculture: diverting and storing water, irrigating during dry seasons, compacting soil surface Urbanization: paving soil surfaces (To name a few…) Leads to: increased evaporation and runoff, decreased storage – an accelerated hydrologic cycle

Eve’s research interest: Hydrologic pathways through the landscape influence biological and chemical processes Hydrologic flow paths are the connection between land and water systems, affecting the distribution of reactive elements, and modifying the physical, chemical, and biological factors that control the fates and forms of reactive elements. Much work has been done to characterize the different hydrological flow paths within the terrestrial system, including… Inspired by Dunne, 1980

Mississippi River Decisions about nutrient inputs at the local scale have consequences at the regional scale N, P The impact on elemental cycles occurs and is most easily seen at local and regional scales. N and P inputs to ag fields affect the Gulf of Mexico by causing increased primary productivity and eutrophication. However, one of the greatest uncertainties at this scale is in understanding how these elements travel across the landscape; how do they react as they move from the land to the water system? What is the pathway they take? Gulf of Mexico Dead Zone Source: www.nasa.gov

Hillslope tracer experiments in the Boulder Creek Critical Zone Observatory

CLIMATE AND ECOLOGICAL ZONES

59% 70% 85% 85% Alpine Sub-alpine High Montane Montane Figure 3: Amount and type of precipitation collected as part of the National Atmospheric deposition program (NADP) at for locations within Boulder Creek Watershed 59% 70% 85% 85% Rory Cowie, in prep

Problem: Increases in Nitrogen Deposition at High Elevation from Human Activities Eastern slope > 2x western slope, 75% from human activities (Baron et al. 2000) NH4NO3

The “Reactor on a Slope” Overarching research question of Boulder Creek CZO Erosion = lowering, due to divergence of sediment flux (difference b/w inputs and outputs)

Research Questions How do changes in dominant flow paths along a montane hillslope profile affect the fate and transport of deposited N? (2) Do hydro-biogeochemical interactions differ on N- and S-facing slopes?

Ecosystem pools and fluxes measured

Ecosystem pools and fluxes measured

Snowmelt Tracer Study, 10 April 2010 SLOW application rate 40 days of daily sampling, 14 days of event-based sampling LiBr + K15NO3- - N in water SNOW

Some flow hypotheses for the South-facing slope ? ?

Some flow hypotheses for the South-facing slope Potential for hydrologic connectivity at depth, movement laterally, and reaching stream in spring Low potential for hydrologic connectivity, unless there is a huge storm event

Some flow hypotheses for the North-facing slope Snow accumulates, one big spring melt event Freeze and thaw occurs in snowpack and soil throughout winter ? ?

Some flow hypotheses for the North-facing slope Snow accumulates, one big spring melt event Freeze and thaw occurs in snowpack and soil throughout winter Potential for hydrologic connectivity at depth, movement laterally, reaching stream. Potential for hydrologic connectivity and more preferential flow

How we start to infer hillslope transport processes from tracer data

Looking through the soil profile, by the creek

One of the challenges of hillslope hydrology in snow-dominated catchments: Modeling snowpack dynamics and melt water flux Williams et al., 2010

A first attempt at modeling melt water flux TRACER EXPT Upper S-facing slope Upper N-facing slope Melt water flux (m3 m-2 day-1) Work in progress with Bob Anderson

S-facing slope N-facing slope Current challenge in hillslope hydrology: understanding flow paths and stream response at elevations where snowpack is variable in time and space and predicting how these areas will respond to long-term climate change. Water runs downhill, but there is still a lot to be done! S-facing slope N-facing slope