National Institute for Water Resources, Water Resources Research Institute Program
3/1/2005 - 2/28/2006
As populations increase and the demand for water rises, so does the need for informed sustainable resource management. In Hawai‘i, one particular concern is that land use and vegetation change are adversely affecting groundwater recharge and thus freshwater supplies. While parcels of native forest are protected through the designation of water catchment areas, non-native plants threatening these forest parcels are believed to transpire more than their native counterparts or alter forest structure in a way that reduces groundwater recharge. However, these hypotheses have not been rigorously tested, and the hydrology of the typical Hawai‘i watershed accounting for plant water uptake and growth has yet to be modeled.
Our research objective was to test the specific hypothesis that vegetation change alters groundwater recharge by affecting the evapotranspiration (ET) term of the fundamental water balance equation. This was based on the hypothesis that many non-native invasive plant species are faster-growing and thus have higher transpiration rates than native species in Hawai‘i, as suggested indirectly by physiological studies (Z. Baruch and G. Goldstein, 1999, Leaf construction cost, nutrient concentration, and net CO2 assimilation of native and invasive species in Hawaii, Oecologia 121:183–92; R. Pattison, G. Goldstein, and A. Ares, 1998, Growth, biomass allocation and photosynthesis of invasive and native Hawaiian rainforest species, Oecologia 117:449–459; S. Cordell, R.J. Cabin, and L.J. Hadway, 2002, Physiological ecology of native and alien dry forest shrubs in Hawaii, Biological Invasions 4:387–396). However, scaling up these physiological rates to estimate native and non-native stand water use has not been performed in Hawai‘i, and how these contribute to large-scale water balance is unclear. Architectural differences are also thought to affect ET within the forests, since it has been observed that disturbed nonnative forest communities often have decreased understory and aerodynamic roughness, resulting in increased evaporation due to increased wind speed.
Since the inception of this project in 2004, focus has shifted from water balance modeling to the modeling of plant transpiration for dominant canopy and understory species in native and non-native forests. The objective was to generate species-specific transpiration models based on meteorological variables, soil water potential, and leaf area. These models were then combined with vegetation data to estimate transpiration for any given stand.