Hydrology of forest ecosystems in the Honouliuli Preserve: Implications for groundwater recharge and watershed restoration (Hawaii)

The main objectives of this study were (1) to quantify and explain differences in the components of the water cycle among forest stands dominated by non-native tree species and (2) to estimate the potential of these stands to recharge groundwater. These forest stands were planted in the 1900s to repair the hydrological impacts that deforestation had caused on the watersheds in Hawaii in the 1800s. Rainfall interception and leaf area index (LAI) were measured in stands dominated by each of four species (Casuarina glauca, Fraxinus uhdei, Eucalyptus robusta and Grevillea robusta ). Transpiration, patterns of water uptake and soil moisture dynamics were measured in one stand each dominated by the first three species. The data collected were used to assess the potential of some of the stands to recharge groundwater from May 2001 to April 2002.; A long period of drought was observed between January 2000 and October 2001. The Fraxinus stand exhibited deep water uptake and recovered LAI promptly after the end of the drought. The Eucalyptus stand had relatively deep water uptake but had a very slow recovery of LAI in relation to the other stands. The Casuarina stand had shallow water uptake but it was still able to recover LAI relatively quickly. Transpiration, relative to tree basal area, was similar among the three species after the end of the drought.; Interception ranged from 4 to 29% of rainfall for the year of 1999 and there were no significant differences in interception among stands dominated by different species. Evapotranspiration (ET) rates were 90, 94 and 65% of rainfall for Fraxinus, Eucalyptus and Casuarina, respectively. Thirty two percent of rainfall was lost as surface runoff in the Casuarina stand.; No groundwater recharge was observed in these stands during the period studied. The results suggest that the species chosen for reforestation in Honouliuli were not ideal for restoring and protecting the groundwater resource. It would have been more desirable to use species that are conservative water users instead of fast growing trees which are able to reduce erosion in a short term but which exhibit relatively high ET rates even after 80 years.