RECESS - recession of ground-water discharge and estimating mean ground-water recharge and discharge from streamflow records
RECESS Category: flow models - recession
Introduction to RECESS
It is preferable, but not required, that the record of daily mean streamflow is at least several years in duration. Results are most useful if the programs are executed to give long-term (at least yearly) results. Interbasin comparisons should be made only when the programs are executed for a uniform time period because of climatic variations. The programs work on the basis of the calendar year. In the application of the methods for estimating recharge or discharge, the record should be complete for each year in the period of analysis (365 or 366 daily values per calendar year).
The procedure for defining the recession of ground-water discharge selects several individual periods of continuous streamflow recession (periods during which each pair of subsequent daily values must meet the condition that the first value exceeds or is equal to the second). Periods of reduced precipitation must be long enough that the profile of the ground-water head distribution is nearly stable and ground-water discharge will plot with characteristic linearity or near-linearity on the graph of the logarithm of flow as a function of time. This condition may not be met for ground-water flow systems of very low diffusivity (transmissivity divided by storage coefficient).
The procedure for defining streamflow recession allows for the possibility of nonlinearity in the relation between the logarithm of ground-water discharge and time when the profile of the ground-water head distribution is nearly stable. Nonlinearity can result from variation in saturated thickness, geologic heterogeneities, and attrition of ground water to leakage or evapotranspiration.
The methods for estimation of recharge or discharge are intended for the analysis of flow systems that are driven by areally-diffuse recharge events that can be considered to be roughly concurrent with peaks in streamflow. The stream is the sink (discharge boundary) of the ground-water flow system. Results of these methods may not be reliable for flow systems dominated by leakage to or from regional flow systems, snowmelt runoff, recharge from losing streams, or ground-water withdrawals. The methods for estimating recharge or discharge should be used only if the drainage area is larger than one square mile so that the time base of surface runoff will exceed the time increment of the data (one day). The upper limit of drainage area may depend on the degree of nonuniformity of weather systems. It has been suggested that the unit hydrograph concept should not be applied for basins that are larger than 2,000 square miles and that the upper limit may be much smaller where convective rainfall predominates. The upper limit for the unit hydrograph concept may also apply to the methods described here for estimating recharge and discharge.
In addition to the above, the user should assess the validity of the mathematics described in the study area. For example, the procedure for defining streamflow recession uses the assumption that the recession index is linearly related to the logarithm of flow, and the methods for estimating ground-water recharge and discharge use an empirical equation for determining the time base of surface runoff to estimate the number of days after a peak in streamflow when most flow can be considered ground-water discharge. Output of the programs allows for some degree of adjustment for variation between the real problem and the default mathematics of the programs.
Source, executable codes, and technical support are included with the package.
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