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Appendix E: Finding Landscape Coefficient and Watering Needs of Experimental Garden

Landscape coefficient (Costello et al.)

To find the landscape coefficient, we must estimate the species factor (k_s), the density factor (k_d), and the microclimate factor (k_mc).

The species factor accounts for differences in the water needs of different species. The plants are classified as either very low, low, medium, or high water needs, and the classification of the plants in our garden (for our region) are found in the Water Use Classification of Landscape Species. Almost all the plants in our garden are in the low water use category, for which k_s = 0.1-0.3, so we estimate a k_s of 0.2 for our garden.

The density factor accounts for differences in the density, or leaf area of different plantings. The density factor depends on the canopy cover of the plantings (how much of the ground they cover) and the height of the plantings. Less than 90% canopy coverage constitutes a low density planting (k_d = 0.5-0.9). From our projections of the plot when mature, the canopy cover will deifinitley be less than 90%. However, because there will be some trees (which increase the density) k_d is estimated to be at the high end of the low density category. Thus, we estimate k_d = 0.9.

The microclimate factor accounts for differences in microclimate between different locations in the landscape. Plantings located to the north or north-east of buildings, in shaded areas, or protected from winds have lower k_mc factors because these features tend to lower evaporative conditions. Sites exposed to direct wind, heat inputs from nearby sources or reflected light are considered to have a high k_mc. Because the site of the garden (to the east of Garrett House) is on the east side of a building and partially shaded and protected from wind by the wall around Garrett House but also experiences some wind and reflected light from the walkways, cars, buildings and windows nearby, we estimated that the microclimate was average (these factors cancel each other out). The k_mc for an average microclimate is 1.0.

The landscape coefficient in then found by simply multiplying k_s k_d k_mc.

0.2 0.9 1.0 = 0.18 = 18% of reference evaoptranspiration

Actual water needs

Water needs for our experimental plot were estimated from evapotranspiration and precipitation data for 12/99-11/00. Landscape coefficient of experimental plot was calculated to be 18% ET_o. Precipitation was subtracted from water needs for each month (as rain will take care of some water needs). Then the positive values (water that is needed to supplement rain) were added together to get the yearly supplemental water needs of our plot. This number is for last year, and is subject to change from year to year with changing weather, and is thus meant to serve as an estimate for the yearly water needs of our plot.

0.425 ft water/year 3120 ft² (area of plot) = 1326 ft³ water/year 7.48 gal/ft³ = 9918 gal/year

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