A MODELING PARADIGM FOR WATER BALANCE IN CENTRAL OREGON FEN ENVIRONMENTS

Pumping in fens in central Oregon, fed by groundwater under artesian pressure, serves to water cattle. On average, the pumping rates are approximately 0.009 l/s (0.78 m³/d) for the 75 days of the grazing season. The fens are sloped, with a gradient typically about 2°, but may be up to about 5°. We modeled potential impacts of this pumping.

We constructed a generic fen model with MODFLOW 2000 taking the slope range into account and using the average evapotranspiration rates for the area balanced with corresponding aerial recharge. We assumed the recharge rate to be 400 mm/y for the model. The modeled fen covers an area of 2300 m² (0.23 hectares) and has an evaporation rate of 1800 mm/y, as determined in the field. We utilized a pumping rate of 0.009 l/s to simulate use during the 75 day grazing season. We also modeled a rate of 0.09 l/s for comparative purposes.

The local stratigraphic sequence is basically 3-layered; 1 m of peat on top (hydraulic conductivity (K) =0.2 m/d) with a low-permeability base (K=0.001 to 0.0001 m/d), a 1 m pumice layer (K=12 m/d), and 10 m of volcanics below (K=50 m/d). Storage coefficient is universally 0.0001 per meter and specific yield is 0.2 in layer 1. There is an impermeable confining layer between the pumice and the volcanics.

The model represents spring flow from a fully confined volcanic layer having constant heads around the periphery. A hole, simulating a fault or other structural discontinuity in the confining layer at the upper end of the fen allows flow from the volcanic layer. The size of the hole and constant heads are adjusted to produce an average of the spring flow rates measured in the field; approximately 0.35 l/s. This spring flow maintains the fen environment.

A negative impact would be assumed to occur if drawdown from 75 days of pumping lowered the water table in the upper layer by 5 cm or more. In general, pumping at the observed use rate of 0.009 l/s does not produce this level of impact, regardless of whether the peat basal layer has hydraulic conductivity of 0.001 or 0.0001 m/d. Maximum drawdown in the upper layer in this instance is 2.2 cm. However, if the pumping rate is increased by an order of magnitude to 0.09 l/s, drawdown within the fen area is everywhere greater than 5 cm, with a maximum of 23 cm at the pumping center. Changing the slope from 2 degrees to 5 degrees has little to no effect.