While plant invasion is a globally recognized threat to biodiversity, the impacts incurred on ecosystem processes are not as apparent (Nelson 2005). Elucidating the influences that invasive plants have on hydrologic processes is particularly important to ground water modelling and water resource management, as well as watershed management and species conservation.
There is a multitude of research assessing hydrologic processes (groundwater processes, streamflow processes etc.); mostly technical reports from USGS and BWS. Also, there is an extensive literature based around the physiology of Hawaiian native and common invasive plants. Kagawa and Giambelluca (2016) reviewed 150 papers from 1910 to the present that they felt could be classified as either hydrological or ecological studies related to ecohydrology. The driving question was how to “best generalize or contextualize ecohydrological insights drawn from studies across Hawaiʻi’s diverse environments”. This review differs by focusing on research that directly measured one, or more, components of the water cycle. These studies either characterized a forest community or compared between vegetation assemblages. The driving research questions pertained to how native ecosystems function with regards to groundwater recharge, and how these processes might change with land cover change, due to various mechanisms such as grazing ungulates and the following degredation and plant invasion. A total of 15 papers were considered to address the research concern of interest. They spanned 12 field sites on three of the 8 Hawaiian Islands. Noteably, only two of these sites had been measured for multiple components of the water balance.
There are three mechanisms by which plants influence the movement of water in the water cycle: (1) they intercept water, generating a storage pool of water in the canopy that is then lost back to the atmosphere through evaporation, or that can become throughfall, changing the timing of the delivery of water to the soil surface; (2) they alter the physical structure of the soil impacting the pathway of water below the surface, as well as the storage capacity of the soil; and (3) they act as the pathway for water to move from the soil back to the atmosphere, via transpiration (Engott 2011).
The water budget model is a useful framework for determining changes of a “reservoir” by tracking the inputs and outputs of water into a closed system according to the the mass balance approach of Thornthwaite and Mather (1955) (Engott 2011). It is a simple approach that requires only inputs of rainfall, evaporation and a starting soil moisture value (Restom-Gaskill 2004).
Net Precipitation and Interception:
Net precipitation describes the total amount of rainfall that reaches the soil after falling on a vegetated surface. In areas where fog interception contributes a considerable amount of water to the soil, net precipitation can be higher than gross rainfall.
ET is the combined loss effect of liquid water back to the atmosphere. Additionally, it can be partitioned into three pathways; evaporation of water intercepted by the canopy, evaporation from the soil surface; or water evaporated out of a plant through the process of transpiration.
Soil Properties and Infiltration:
Soil hydraulic properties, such as saturation conductivity (Ksat) can be influenced by the organic matter content of the soil.
Currently, water budget models for Hawaiʻi simplify the role of vegetation on hydrologic processes, although Engott (2011) in the most current water budget for Hawaiʻi Island utilized two models, one for pasture and one for forest. By investigating the mechanims by which various land covers, across the diverse climate gradient found in Hawaiʻi, impact the water cycle, resource managers will be able to assess whether these simplifications over- or underestimate ground water recharge rates. Additionally, understanding the varying responses between land cover types to meteorological conditions is crucial for predicting how natural resources might change with climate change. Strauch et al. (2017) used a distributed soil hydrology vegetation model (DSHVM) to show how changes in vegetation cover, rainfall, and temperature would impact stream flow on the Hamakua coast of Hawaiʻi Island. The model employed species level traits for transpiration (i.e. high and low stomatal conductance) determine from some of the literature included in this review, emphasizing the importance of increasing the body of literature that describes species level characterstics that can influence hydrologic processes.
Cross Cutting Conclusions
1)Long-term measurements so that seasonal variation does not confound species differences
2) Varying hydrologic processes dominate in different climates, requiring that hydrologic functioning of ecosystems across the scope of climate diversity are assessed.
3) Additionally, different landcover change threats as well as the various landcover changes that threaten them via plant invasion in order to generalization potential patterns of changes to groundwater recharge across the state
4) More research to clarify mechanisms behind measured differences
Brauman, Kate A., David L. Freyberg, and Gretchen C. Daily. 2010. “Forest Structure Influences on Rainfall Partitioning and Cloud Interception: A Comparison of Native Forest Sites in Kona, Hawai’i.” Agricultural and Forest Meteorology 150: 265–75. doi:10.1016/j.agrformet.2009.11.011
Engott, John. 2011. “A Water-Budget Model and Assessment of Groundwater Recharge for the Island of Hawai‘i.”
Kagawa, Aurora and Thomas Giambelluca. 2016. “Landscapes of Change: An Overview of Hawaiʻi ecohydrology-related research.” Emerginig Issues in Tropical Ecohydrology, Chapman Conference. Poster M-09.
Levia, Delphis F., and Ethan E. Frost. 2006. “Variability of Throughfall Volume and Solute Inputs in Wooded Ecosystems.” Progress in Physical Geography 30 (5): 605–32. doi:10.1177/0309133306071145.
Nelson, Gerald. 2005. “Drivers of Ecosystem Change: Summary Chapter.” In Ecosystems and Human Well-Being: Current State and Trends, 1:73–76
Restom Gaskill, Teresa. 2004. “Hydrology of Forest Ecosystems in the Honouliuli Preserve: Implications for Groundwater Recharge and Watershed Restoration.” University of Hawaii.
Strauch, Ayron M., Christian P. Giardina, Richard A. MacKenzie, Chris Heider, Tom W. Giambelluca, Ed Salminen, and Gregory L. Bruland. 2017. “Modeled Effects of Climate Change and Plant Invasion on Watershed Function Across a Steep Tropical Rainfall Gradient.” Ecosystems 20 (3). Springer US: 583–600. doi:10.1007/s10021-016-0038-3
Wilson, Kell B, Paul J Hanson, Patrick J Mulholland, Dennis D Baldocchi, Stan D Wullschleger, and K B Wilson. 2001. “A Comparison of Methods for Determining Forest Evapotranspiration and Its Components: Sap-Flow, Soil Water Budget, Eddy Covariance and Catchment Water Balance.” Agricultural and Forest Meteorology 106: 153–68