6.1 swb.aml

This AML generates a site water balance grid based on the site water balance (SWB) model. Bucket size, monthly averages of daily potential evapotranspiration (1/10 mm) and montly precipitation sums (mm) are combined to calculate the spatially-explicit site water balance similar to the approach first employed by Grier and Running (1977). Beginning with the first month in fall when precipitation exceeds potential evapotranspiration (after a possible drought period), the difference between precipitation and potential evapotranspiration is summed over 12 months. The running sum is never allowed to exceed the bucket size, and water in excess of the bucket size is presumed to run off. When potential evapotranspiration begins to exceed precipitation the difference is subtracted from water in the bucket, often achieving significant negative values over the course of a year.

Site water balance is an estimate of the water available to plants during a year, and integrates both climatic and soil parameters. The method differs from Grier and Running (1977) in the determination of the appropriate water year, and in not assuming that soils begin the water year with full recharge. See Fig.1 for an example of the site water balance calculations or check here for more illustrations and explanations. The method is also explained in Roberts et al. (1993) and under this web site.

The resulting grid is called "swb". It contains the site water balance estimates (in mm water per year) calculated over large landscapes. Such maps can be used for ecological analysis, since growth, nutrient uptake, and the development of ecosytem (or stand) leave area (SLA) is largely driven by site water balance. See Roberts et al. (1993) for further explanations and interpretations.
 
 

General specifications of the AML:

Command: &r  swb     (at GRID prompt)
Required input: Grids of precipitation, potential evapotranspiration, and col bucket
Output units: Grids of SWB (mm/year)
Speed of calculations: Rather fast
Flexibility of the routine: Medium; user can adjust AML at interface
User interface: Limited, but simple and easy
Known errors:  -
Programmer N.E. Zimmermann
Download: swb.aml       (use: "save link as")
Contact: niklaus.zimmermann @ wsl.ch

 

References:
 

Grier, C.C. & Running, S.W. 1977. Leaf area of mature north-western coniferous forests: relations to site water balance. Ecology 58: 893-899.
 
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Last Updated: 9/08/00
By Niklaus E. Zimmermann