Meteorology and Hydrology at Bisley related data sets

PROJECT DESCRIPTION: Several meteorological parameters are being measured at Bisley since 1993. Correlations between elevation and stream-runoff and rainfall, elevation and air and soil temperature, and between trhoughfall and vegetation types have been found. These relationships are used inhydrologic and nutrient budgets as well as in environmental models .

Rainfall and Stream-runoff

Long-term rainfall and discharge data from the Luquillo Experimental Forest (LEF) were analysed to develop relationships between rainfall, stream-runoff, and elevation. These relationships were then used with a Geographic Information System (GIS) to determine spatially-averaged, mean annual hydrologic budgets for watersheds and forest types within the study area. Model estimates indicate that a total of 3864 mm/yy (444 hm3) of rainfall falls on the forest in an average year. The Tabonuco, Colorado, Palm and Dwarf Forest types receive an estimated annual rainfall of 3537, 4191, 4167, and 4849 mm/yy, respectively. Of the average annual rainfall input, 65% (2526 mm/yr) is converted to runoff and the remainding 35% (1338 mm.yr) is lost from the system by evapotranspiration and other abstractions. In comparison to other tropical forests, the LEF as a whole has more evapotranspiration than many tropical montane forests but less evapotranspiration than many lowland tropical forests.

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Throughfall
Changes in the quantity and quality of precipitation as it passes through vegetative cover are important components of both hydrologic and nutrient budgets.

Throughfall over any period depends on the balance between precipitation, evaporation and canopy storage (Horton, 1919; Leonard, 1967; Rutter et al., 1972). If the watershed is divided into different vegetation types based on similarity in throughfall and stemflow, the total throughfall over the watershed can be expressed as:

(1) Pg = Sum( T n A n )+ Sum (Sm Dm)

Where Pg = total throughfall reaching the ground, Tn = canopy throughfall from vegetation type n, An = area of vegetation type n, Sm = stemflow from stem type m and Dm = number of stems in type m.

Using eqn. (1) to estimate total watershed throughfall becomes a problem of determining the minimum number of vegetation types necessary to describe the system at the required level of accuracy. In one of our studies, measured throughfall was compared with actual canopy and stem conditions to estimate the percentages of throughfall for different time periods was calculated by weighting the average throughfall and stemflow measured in representative areas of each vegetation type by the total area of that vegetation group.

Measurements reported here were made in two of the Bisley Research Watershed of the U.S. Forest Service. These adjacent watersheds drain 13.0 ha of highly dissected mountainous terrain that range in elevation from 265 to 455 m. Both watersheds are covered by Tabonuco type forests and were selectively logged at various times between 1860 and 1940 (Scatena, 1988).

The dominant tree in the watersheds in the Tabonuco (Dacryodes excelsa) which often comprises as much as 35% of the canopy (Wadsworth, 1970). Structurally the forest has three dominant layers, a discontinuous emergent strata, a continuous upper stratum at 20 m, and an understory layer. Leaves are mesophyllous and often covered with epiphytic growth.

Throughfall

Changes in the quantity and quality of precipitation as it passes through vegetative cover are important components of both hydrologic and nutrient budgets.

Throughfall over any period depends on the balance between precipitation, evaporation and canopy storage (Horton, 1919; Leonard, 1967; Rutter et al. , 1972). If the watershed is divided into different vegetation types based on similarity in throughfall and steamflow, the total throughfall over the watershed can be expressed as:

(1) Pg = Sum( T n A n )+ Sum (Sm Dm)

Where Pg = total throughfall reaching the ground, Tn = canopy throughfall from vegetation type n, An = area of vegetation type n, Sm = stemflow from stem type m and Dm = number of stems in type m.

Using eqn. (1) to estimate total watershed throughfall becomes a problem of determining the minimum number of vegetation types necessary to describe the system at the required level of accuracy. In one of our studies, measured throughfall was compared with actual canopy and stem conditions to estimate the percentages of throughfall for different time periods was calculated by weighting the average throughfall and stemflow measured in representative areas of each vegetation type by the total area of that vegetation group.

Measurements reported here were made in two of the Bisley Research Watershed of the U.S. Forest Service. These adjacent watersheds drain 13.0 ha of highly dissected mountainous terrain that range in elevation from 265 to 455 m. Both watersheds are covered by Tabonuco type forests and were selectively logged at various times between 1860 and 1940 (Scatena, 1988).

The dominant tree in the watersheds in the Tabonuco ( Dacryodes excelsa ) which often comprises as much as 35% of the canopy ( Wadsworth, 1970). Structurally the forest has three dominant layers, a discontinuous emergent strata, a continuous upper stratum at 20 m, and an understory layer. Leaves are mesophyllous and often covered with epiphytic growth.

Air and Soil Temperature

The relationship between mean air temperature and elevation is a required parameter for some environmental models such as Zelig. Mean air and soil temperature measurements of 10 sites located along a windward elevation gradient from 153 to 1011 meters were used to develop relationships between mean air and soil temperature of and elevation. The regressions performed showed a linear relationship between both air and soil mean temperature and elevation. The equations:

(2) Mean Air Temperature (in C) = 26.4 -(0.00558 * elevation in meters) and

(3) Mean Soil Temperature (in C) = 25.6 - (0.00543 * elevation in meters)

best fit these relationships. The equation that best fits the mean soil temperature - elevation relationship includes all the stations. In contrast, the best equation for the mean air temperature - elevation relationship excluded both station located at Sabana.