# alpha\_bf The baseflow recession constant is a direct index of groundwater flow response to changes in recharge (Smedema and Rycroft, 1983). Values vary from 0.1-0.3 for land with a slow response to recharge to 0.9-1.0 for land with a rapid response. Although the baseflow recession constant may be calculated, the best estimates are obtained by analyzing measured streamflow during periods of no recharge in the watershed. It is common to find the baseflow days reported for a stream gage or watershed. This is the number of days for base flow recession to decline through one log cycle. When baseflow days are known, the alpha factor can be calculated: $$ α\_{gw} = \frac{1}{N} \* ln⁡\[\frac{Q\_{gw,N}}{Q\_{gw,0}} ]=\frac{1}{BFD} \* ln⁡\[10]=\frac{2.3}{BFD} $$ where $$α\_{gw}$$ is the baseflow recession constant, and *BFD* is the number of baseflow days for the watershed. > Smedema, L.K. and D.W. Rycroft. 1983. Land drainage - planning and design of agricultural drainage systems. Cornell University Press, Ithaca, NY. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://swatplus.gitbook.io/io-docs/introduction-1/aquifers/aquifer.aqu/alpha_bf.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.