Water budgets



Accurate assessment of water budgets is critical for effective management of water resources, especially on small, densely-populated islands with extremely limited storage capacity such as Puerto Rico. A water budget defines a balance between inputs, outputs, and storage. The water budgets described herein provide a generalized summary of the inputs, extractions, and outputs from four watersheds in and near the Luquillo mountains using rainfall, runoff, and public-supply extraction data as well as estimates of groundwater losses and inputs such as cloud drip and infiltration from septic tanks. Mean annual rainfall accumulation during a 7-year study (1991 to 1997) ranged from 1,722 mm in the Canóvanas watershed, to 4,235 mm in the Icacos and Mameyes watersheds; the Cayaguás watershed had 2,172 mm. Combined runoff, groundwater flow and withdrawals ranged from 47 to 73 percent of inputs (combined rainfall, cloud drip and septic tank infiltration). Evapotranspiration, calculated as the water budget residual, amounted to 27, 40, 44, and 53 percent of total moisture inputs in the Icacos, Cayaguás, Mameyes, and Canóvanas watersheds, respectively.

Water Withdrawn From the Luquillo Experimental Forest, 2004

Crook, Kelly E.; Scatena, Fred N.; Pringle, Catherine M. 2007. Water Withdrawn From the Luquillo Experimental Forest, 2004. U.S. Department of Agriculture, Forest Service, International Institute of Tropical Forestry. Gen. Tech. Rep. IITF-GTR-34.

This study quantifies the amount of water withdrawn from the Luqillo Experimental Forest (LEF) in 2004. Spatially averaged mean monthly water budgets were generated for watersheds draining the LEF by combining long-term data from various government agencies with estimated extraction data. Results suggest that, on a typical day, 70 percent of water generated within the forest is diverted before reaching the ocean. This is up from an estimated 54 percent in 1994. Analysis showed that up to 63 percent of average monthly stream runoff is diverted from individual watersheds during drier months. Watersheds with large water intakes have the most dramatic decrease in streamflow, particularly the Río Espiritu Santo watershed, where 82 percent of median flow is diverted.

Estimating fog deposition at a Puerto Rican elfin cloud forest site: comparison of the water budget and eddy covariance methods

Holwerda, F., R. Burkard, W. Eugster, F. N. Scatena, A. G. C. A. Meesters,
and L. A. Bruijnzeel (2006), Estimating fog deposition at a Puerto
Rican elfin cloud forest site: Comparison of the water budget and eddy
covariance methods, Hydrol. Processes, 20, 2669– 2692.

The deposition of fog to a wind-exposed 3 m tall Puerto Rican cloud forest at 1010 m elevation was studied using the water budget and eddy covariance methods. Fog deposition was calculated from the water budget as throughfall plus stemflow plus interception loss minus rainfall corrected for wind-induced loss and effect of slope. The eddy covariance method was used to calculate the turbulent liquid cloud water flux from instantaneous turbulent deviations of the surface-normal wind component and cloud liquid water content as measured at 4 m above the forest canopy. Fog deposition rates according to the water budget under rain-free conditions (0Ð11 š 0Ð05 mm h1) and rainy conditions (0Ð24 š 0Ð13 mm h1) were about three to six times the eddy-covariance-based estimate (0Ð04 š 0Ð002 mm h1). Under rain-free conditions, water-budget-based fog deposition rates were positively correlated with horizontal fluxes of liquid cloud water (as calculated from wind speed and liquid water content data). Under rainy conditions, the correlation became very poor, presumably because of errors in the corrected rainfall amounts and very high spatial variability in throughfall. It was demonstrated that the turbulent liquid cloud water fluxes as measured at 4 m above the forest could be only ¾40% of the fluxes at the canopy level itself due to condensation of moisture in air moving upslope. Other factors, which may have contributed to the discrepancy in results obtained with the two methods, were related to effects of footprint mismatch and methodological problems with rainfall measurements under the prevailing windy conditions. Best estimates of annual fog deposition amounted to ¾770 mm year1 for the summit cloud forest just below the ridge top (according to the water budget method) and ¾785 mm year1 for the cloud forest on the lower windward slope (using the eddy-covariance-based deposition rate corrected for estimated vertical flux divergence). Copyright  2006 John Wiley & Sons, Ltd.
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