Masters Theses

Date of Award

8-2003

Degree Type

Thesis

Degree Name

Master of Science

Major

Geography

Major Professor

Carol P. Harden

Abstract

Infiltration rates of soils in the Luquillo Experimental Forest (LEF) of Puerto Rico were measured in situ in January 1998 using a McQueen portable rainfall simulator. Because of the significance of infiltration in watershed hydrology, measuring infiltration rates and understanding their spatial variability are important components of hydrologic and environmental research. This study explores the relationship between infiltration rates, antecedent volumetric moisture and bulk density and the four primary vegetation types in the Luquillo Experimental Forest. The four vegetation types are tabonuco forest, palm forest, colorado forest and cloud forest. The relatioinship between infiltration rates, antecedent moisture and bulk density are also examined on the basis of topographic position. The classes of topographic position used for this study are ridge, slope, cove, and riparian zone.

This study also explores the relationship between infiltration rates and soil associations and the relationship between infiltration rates and other site parameters.

We performed 54 infiltration experiments throughout the LEF. Although strong statistical relationships were not established between infiltration rates and most of the observed environmental parameters and site factors, some interesting trends emerged from the data.

A large range of infiltration rates was observed in the Luquillo Experimental Forest and in the Bisley 1 Experimental Watershed. Across the Luquillo Experimental Forest infiltration rates ranged from a low of 0.0 mm/hr to a high 98.7 mm/hr. In the Bisley 1 watershed, the only statistically significant relationship found between infiltration rate and other site factors was that between infiltration rates and topographic position. There, infiltration rates of topographic cove areas were significantly different from those on slopes and ridges. Sites in the cloud forest appeared to have lower infiltration rates than sites in other types of forest, but the difference was not significant for the sample size (four cloud forest sites) used in this initial study.

By using the infiltration values generated from this initial study to obtain a reasonable estimate of the population variance, I was able to calculate the number of sample runs required to obtain satisfactory results at the 0.05 level of significance using a Student's t-test. Thirty-four infiltration runs in each of the four forest types of topographic position classifications would provide sufficient resolution. A focused research comparing the infiltration rates of one classification of topographic position, e.g. ridge, across the four forest types may reveal patterns of infiltration rate variability related to forest type. The data we collected began to suggest elevation and/or forest type as a control or indicator of infiltration ranges. Yet, the relatively few repetitions of sample runs we were able to perform in each forest type and topographic position limited the type of analysis that could be applied to the data. More infiltration sample runs would be required to make this research more statistically robust. Given a generous amount of time for infiltration rainfall similator experiements and a GIS to target all combinations of topographic position, forest type and soil type, it may be possible to construct a GIS, table, or map that would allow one to make a reasonably accurate estimate of the range of infiltration anywhere in the LEF.

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