INCOMPLETE DRAFT

Catchment Water Balance

Instructor: Jim McNamara

Phone: 426-1354

Email:


1. Learning Objectives:

Students will:

-Explore the variability of water balance components across catchments in differing hydroclimatic regions

2. Problem

The goal of this project is to conduct a comparative water balance exercise using data from Reynolds Creek, Dry Creek, and HJ Andrews sites.

3. Background

Conservation of mass is a fundamental physical concept in hydrologic science. In minus out equals a change in storage. In other words, if the flow rate from a hose into a bucket exceeds the flow rate leaving the bucket from a drain the water level in the buckets rises. Watersheds behave in a similar fashion. Watersheds receive water as rain (R), snowmelt (SN), or groundwater discharge (GWin) , and release water as streamflow (Q), evapotranspiration (ET), or groundwater recharge (GWout). If inflows exceed outflows, the watershed becomes wetter, that is to say soil moisture increases, groundwater levels rise, lake levels rise, or some other form of water storage (S) increases.

IN - OUT = Change in storage (1)

(R+SN + GWin) – (Q + ET + GWout) = DS/Dt (2)

Common tasks of a hydrologist are to measure or model the various components of the water balance equation. Precipitation and streamflow are fairly straightforward to measure. Evapotranspiration and groundwater are not.

3. The Watersheds

We will obtain data Dry Creek near Boise, Reynolds Creek in the Owyhee mountains, and HJ Andrews forest in Oregon. Dry Creek and Reynolds Creek share similar climates, while Andrews is in a wet, coastal environment. See class website for papers on each watershed.

4. Instructions

The class will be split into three teams. Each team will be responsible for conducting all analyses for one of three watersheds including Dry Creek, Reynolds Creek, and HJ Andrews. Each team will share reduced data with other teams for comparative analyses.

Data Tasks for each team

1. Download all Streamflow and Precipitation data for your watershed for at least 5 years from the following sites

-Lookout Creek in HJ Andrews

-Dobson Creek in Reynolds

-Lower Gauge in Dry Creek

2. Download DEM’s for all watersheds

3. Delineate the watershed and determine drainage area using a tool of your choice

4. Calculate total annual precipitation over the watershed for each year using a method of your choice (See Dingman chapter 4).

5. Calculate the depth of annual streamflow for each year.

6. Estimate ET for the watershed using a water balance approach.

7. Present results of steps 4-6 in a table to share with other groups.

Analyses

1. Calculate ratios of annual Q/P and ET/P for each watershed.

2. Plot the results from each watershed on a Budyko diagram.

Questions

1. How does Q/P and ET/P vary across watersheds?

2. How do these ratios vary across years?

3. Comment on the potential sources of error when estimating ET by the water balance approach.

4. Comment how the annual water balance might change in a warming climate.

5. Where does your catchment fall on the Budyko Curve?