The response of different hydrologic processes under changing land use/land cover and climate in Mākaha watershed, O'ahu

Abstract

This study was focused on understanding the response of different hydrologic processes to changes in land use/land cover and climate in Mākaha watershed. Rainfall, throughfall, and stemflow were measured between 2006 and 2010 to determine the canopy interception of three non-native tree species (Strawberry guava, Christmas berry, and Coffee). The Distributed Hydrology Soil Vegetation Model (DHSVM) was evaluated in simulating the hydrologic processes and to assess the impact of additional groundwater development on the Mākaha streamflow. A trend analysis was performed using the longterm air temperature data across the island of O'ahu. To assess the sensitivity of streamflow and evapotranspiration to climate warming, a sensitivity analysis using DHSVM was also conducted. Mean throughfall was the lowest (48.1%) under Strawberry guava and the highest (61.2%) under a mixture of Christmas berry, Strawberry guava, and Java plum. However, stemflow was the highest for Strawberry guava (34.8%) and the lowest under the mixture of Christmas berry, Strawberry guava, and Java plum (4.65%). The canopy interception losses were between 18 and 38% of the gross rainfall. The estimated annual groundwater recharge was 31% of the gross rainfall. Results of this study indicate that 11% of streamflow decline over the period 1991-2009 may be attributed to groundwater pumping. Decreasing trend in rainfall and increasing trend in the fraction of the gross rainfall converted into evapotranspiration could have been the other causes behind the streamflow decline. Analysis of air temperature (1949-2007) shows an increasing trend. Changes in island-wide maximum and minimum temperatures occurred in the year 1961 and 1977, respectively. Increases in minimum (0.43 oC/decade) and maximum temperatures (0.29 oC/decade) are higher than those at the global level (0.18 oC/decade, since 1979). The climate change sensitivity analysis indicates that the changes in precipitation had significant impact on streamflow and evapotranspiration compared to temperature. However, temperature increase coupled with decrease in precipitation, showed significant effect on all hydrological components of the watershed. Additionally, changes in leaf conductance and leaf area index with increasing CO2 concentration significantly affect evapotranspiration and subsequently on streamflow. Further study is needed to quantify the changes in vegetation characteristics under elevated CO2 concentrations.