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Chapter 3: Hydrological Processes in Riparian Areas
Water Balance
- The water balance can be used to summarize the amount of water that is cycling from the atmosphere, across the land surface, into the ground, through plants, into the ocean or other large water body, and back to the atmosphere through evaporation and transpiration.
- This convenient method of bookkeeping is a good framework for understanding hydrologic processes.
- EXAMPLE: The Walnut Gulch Experimental Watershed (an ephemeral tributary watershed) provides an annual surface water balance (figure below).
- Average annual precipitation input is 350 mm.
- Approximately 327 mm is detained on the surface for subsequent infiltration. The infiltrated moisture is either evaporated or transpired by vegetation back to the atmosphere.
- Approximately 23 mm of the incoming precipitation is in excess of that which is intercepted and/or infiltrates and becomes runoff. As the runoff moves over the land surface and into dry alluvial channels, 20 mm of transmission losses occur and less than 1 mm of surface runoff is measured at the watershed outlet.
- The 20 mm of transmission losses result in some ground water recharge, some evaporation, and some evapotranspiration from vegetation along the stream channels. Quantities for ground water recharge, evaporation and evapotranspiration are not shown because their quantification is difficult and very site specific.
Figure 3.10. A surface water balance for the Walnut Gulch Experimental Watershed in Southeastern Arizona..
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