Calculating Available Water

Ben Houston, GroundPoint Engineering

In order to estimate how much power a dam can produce, you need to estimate how much water may be available for power production. A simple way to start is to estimate total average flow using the drainage area ratio method. This method approximates the total flow coming from the upstream watershed using the formula

Q = A * Y

Where Q (the total flow) is equal to the watershed area (A) multiplied by the watershed yield (Y). We have implemented this in an online calculator that will estimate the watershed area of any point you like, and calculate the approximate average flow. Flow (Q) is calculated in cubic feet per second (cfs), area (A) is calculated in square miles (sqmi), and we use a default value of for Yield (Y) of 1.6 cfs/sqmi.

Some streams have flow gages installed and the total flows can be estimated directly from gage measurements.

It’s critical to understand that the total estimate flow in a stream is NOT the same as the flow that may be available for hydropower production. That flow will need to be reduced by some amount to account for required “in-stream flow” requirements, or “environmental flow” in order to maintain water quality and healthy ecosystem function.

It’s also important to acknowledge that there are many factors that determine how much water flows in a stream (these factors are universal in nature and not particular to any single stream):

    • Precipitation: The greatest factor controlling streamflow, by far, is the amount of precipitation that falls in the watershed as rain or snow. However, not all precipitation that falls in a watershed flows out, and a stream will often continue to flow where there is no direct runoff from recent precipitation.
    • Infiltration: When rain falls on dry ground, some of the water soaks in, or infiltrates the soil. Some water that infiltrates will remain in the shallow soil layer, where it will gradually move downhill, through the soil, and eventually enters the stream by seepage into the stream bank. Some of the water may infiltrate much deeper, recharging groundwater aquifers. Water may travel long distances or remain in storage for long periods before returning to the surface. The amount of water that will soak in over time depends on several characteristics of the watershed:
    • Soil characteristics: In Georgia, clayey and rocky soils of the northern areas absorb less water at a slower rate than sandy soils, such as in Georgia’s Coastal Plain. Soils absorbing less water results in more runoff overland into streams.
    • Soil saturation: Like a wet sponge, soil already saturated from previous rainfall can’t absorb much more … thus more rainfall will become surface runoff.
    • Land cover: Some land covers have a great impact on infiltration and rainfall runoff. Impervious surfaces, such as parking lots, roads, and developments, act as a “fast lane” for rainfall – right into storm drains that drain directly into streams. Flooding becomes more prevalent as the area of impervious surfaces increase.
    • Slope of the land: Water falling on steeply-sloped land runs off more quickly than water falling on flat land.
    • Evaporation: Water from rainfall returns to the atmosphere largely through evaporation. The amount of evaporation depends on temperature, solar radiation, wind, atmospheric pressure, and other factors.
    • Transpiration: The root systems of plants absorb water from the surrounding soil in various amounts. Most of this water moves through the plant and escapes into the atmosphere through the leaves. Transpiration is controlled by the same factors as evaporation, and by the characteristics and density of the vegetation. Vegetation slows runoff and allows water to seep into the ground.
    • Storage: Reservoirs store water and increase the amount of water that evaporates and infiltrates. The storage and release of water in reservoirs can have a significant effect on the streamflow patterns of the river below the dam.
  • Water use by people: Uses of a stream might range from a few homeowners and businesses pumping small amounts of water to irrigate their lawns to large amounts of water withdrawals for irrigation, industries, mining, and to supply populations with drinking water.

For more information on estimating stream flows see the following references:




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