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The Effects of Forest Cover on Baseflow Characteristics of Streams in the Mountainous Interior of Puerto Rico

Project Number: 2516-DKQ00
Project Chief: Jesús Rodríguez-Martínez
Cooperators: Department of Natural and Environmental Resources
Period of Project: FY 2008-2011

Introduction

Studies of the effects of land-use changes on surface runoff have been a major field of research in hydrology during the last two decades. In particular, several studies have been undertaken to assess the effect of various levels of forest cover on surface-runoff and baseflows. Although the relation between streamflow and forest cover is rather complex and unpredictable because of its dependency on factors such as stage of forest growth, land-surface slope, and geology, the general effects of reducing forest cover (deforestation) or increasing forest cover (afforestation) has been to increase and reduce stream runoff , respectively (Bruijnzeel, 1990).

Results of studies conducted by the United States Geological Survey (USGS) in Puerto Rico to assess the potential water-resources development for the municipalities of Caguas, Carolina, Ponce, Mayagüez, and Comerío indicate that a direct relation exists between greater baseflows and the occurrence of a well-developed forest cover (mature forest cover) in the highlands of the mountainous interior. In these cases, a high rate of precipitation, generally exceeding 70 inches per year (in/yr), is the norm. An example of greater baseflow-generating capacities, seemingly resulting from the presence of a well-developed forest cover (mature forest), is the uppermost area of the Río Turabo basin in the municipality of Caguas, in the northcentral Puerto Rico (Gómez-Gómez and others, 2001). Similar associations of greater baseflow, inferred from higher low-flows, with well-developed forest cover were also encountered in the uppermost parts of the municipios of Mayagüez and Ponce (Rodríguez-Martínez and others, 2003; Rodríguez-Martínez and others, 2005). The baseflow rates at Caguas have been estimated to range from 450 to 550 gal/min-mi2 (equivalent effective recharge of 13 to 17 in/yr). The baseflows at rivers in the municipio of Mayagüez with a mature forest at their headwaters (uppermost portions of their drainage basins) have been estimated to range from approximately 285 to 500 gal/min-mi2 (9 to 15 in/yr in equivalent effective recharge). Similarly, low flow values at the high-elevation, well-forested watersheds in the municipio of Ponce range from 240 to as high as 700 gal/min-mi2 (an effective recharge ranging from 7 to 20 in/yr) (Rodríguez-Martínez and others, 2005).

Baseflows estimated for subbasins in the municipios of Carolina and Comerío are significantly lower than those at similar high elevations in the municipios mentioned above (Rodríguez-Martínez and others, 2002; Rodríguez-Martínez and others, 2001). The highest stream low-flow rate measured in the highlands of Carolina was about 45 gal/min-mi2 (the equivalent of an effective recharge less than 2 in/yr). Most of the low-flow rates measured in the municipio of Comerío were equal to or less than 45 gal/min-mi2. These lower stream low flows in Carolina and Comerío may be due to the more fragmented (discontinuous) forest cover in the highlands of these two municipios than in the municipios of Caguas, Ponce, and Mayagüez as indicated by field reconnaissance and examination of modern aerial photographs. The deforestation in the municipio of Comerío seems to be mainly the result of past agricultural practices, particularly the intense cultivation of tobacco in the predominantly steep slopes, according to local people and historical agricultural records. Although agricultural practices may had been influential in the past, considerable urban growth seems to be the main recent cause of forest cover loss in the highlands of Carolina.

Johnson and Lehmann (2006) and Koichiro and others (2001) conducted studies that could shed light into the mechanisms or processes that may affect the baseflow characteristics of a basin. In these two studies the role of stemflow as a major source of infiltration into the soil was examined. Johnson and Lehmann (2006) determined that infiltration of stemflow, which results from the partition of rainfall by the tree canopy, is enhanced by root-induced preferential flow. According to Johnson and Lehman (2006), the amount of precipitation partitioned by trees to stemflow may range from less than 1 to as much as 22 percent of incident rainfall in a rainfall regime between 24 and 200 inches per year, according to Johnson and Lehman (2006). Also, Johnson and Lehmann (2006) found that factors such as tree species (including size variations within the same specie), morphology of leaves and branches (the so called canopy architecture) may influence rain partitioning into stemflow and the aerial extent and depth of the root-induced preferential flow zone. Wu and others (2006) through a modeling study determined the existence of a predominant inverse relation between evapotranspiration and elevation at the Luquillo Experimental Forest in northeastern Puerto Rico. This inverse relation of evapotranspiration with elevation might be ascribed to general cooling of the climate which in turn results in a general reduction of water consumption by vegetation with elevation. However, the study by Wu and others (2006) did not relate this inverse relation between evapotranspiration and elevation with baseflow characteristics. Taniguchi and others (1996) determined that, although the ratio of stemflow to precipitation was small in the water balance of a forested basin, the effect of stemflow on groundwater recharge was relatively large.

In addition, the results of a study conducted to evaluate how current and past land-uses may have affected the nutrient load of Lago Cidra in east central Puerto Rico indicated the order of magnitude of the effects of deforestation and afforestation on the stream low-flows (Ramos-Ginés-1997). A low-flow rate of about 210 gal/min-mi2 (equivalent to about 6 in/yr of effective recharge) was estimated for a predominantly forested watershed in the municipio of Cidra. Significantly lower low-flow rates of 45 to 90 gal/min-mi2 (1 to 2 in/yr of equivalent recharge) were estimated for the municipio of Cidra from agricultural areas with none to minimum forest cover. A trend analysis for the period 1972-1998 of the annual minimum 7-day low flows at the Río Inabón (on the leeward-facing slope of Puerto Rico), the Río Grande de Manatí (windward facing slope), and Río Tanamá (windward- facing slope) indicate that increases of about 90 gal/min-mi2, 110 gal/min-mi2, and 140 gal/min-mi2 have occurred for the entire period, respectively (F. Gómez-Gómez, USGS, written comm.., 2007). These increases in low-flow rates, that can be similarly expressed as increases in baseflows, may be reasonably attributed to continuous reversion into forest of agricultural land in the watersheds of these rivers.