A watershed of a river is 6220 mi^2 in area. According to NOAA, the ‘normal’ ann

Question

A watershed of a river is 6220 mi^2 in area. According to NOAA, the ‘normal’ annual precipitation over the period 1971-2000 is roughly 12 inches, averaged across the whole basin. The average discharge of the River, over the last 71 years was 38.8 cfs, but over the same time period as the precipitation estimate (1971-2000) it is only 33.4 cfs, and over the last 10 years it has been 28.0 cfs. Based on data collected, assume that the net incoming shortwave radiation (incoming minus outgoing), averaged over a full year, is 74 J cm^-2 hr^-1 and the average net outgoing longwave radiation (outgoing minus incoming) is 32 J cm^-2 hr^-1. Finally, assume that the annual average Bowen Ratio, the ratio of sensible heat flux to latent heat flux, is 5 over the desert shrub land that dominates the basin.

1.a. Estimate the average annual e 1.a. Estimate the average annual evapotranspiration from the basin in km of water. Use the energy budget equation: R-H-G-LE-p(ET) where is the latent heat of vaporization (assume 2260 kJ/kg), is the density of water, ET is evapotranspiration (m/yT), Ra is net radiation (remember to include both long and short wave, units of J m yr and H is sensible heat flux (also Jmyr). We will assume that soil heat flux (G) is net 0 over the course of a year. 1.b. Take your ET estimate and combine it with the precipitation and runoff data to estimate the average annual groundwater recharge in the basin in km3. Use the annual runoff estimate from the 1971-2000 data. In this case, we can formulate the water budget as: where P is precipitation, ET is evapotranspiration, R is runoff, and GW is groundwater recharge 1.c How much is your estimate of groundwater recharge affected if you use the lower, 2002- 2012 estimate of annual runoff from the Rio Puerco? Which is the dominant pathway in this basin - runoff or ET?

Explanation / Answer

Answer:

Evapotranspiration can be calcuated as follows;

Rn -H-G= Lp(ET)

ET= (Rn -H-G)Lp

Where, ET= Evapotranspiration, Rn= net radiation, H= sensible heat flux, G, soil heat flux= 0 (given), L= 2260 kJ/kg, p= 1g/cm3

Now put the values in above equation to get.

ET= (106)-11300-0 /2260 =- 4.95

To get, Groundwater recharge we apply following formula;

GW= P(ET+R)

Where, GW= Groundwater recharge, P= precipitation, ET=Evapotranspiration, R= Runoff

GW= 12 ( - 4.95 + 5.4)

GW= 12(0.45) = 5.4 cfs

And Annual ground water recharge are about 5.4* 6220= 33588km3

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