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An Ecohydrological Approach to Managing Intermittent and Ephemeral Streams on Department of Defense Lands in the Southwestern United States
Dr. Lainie Levick | University of Arizona
Ephemeral and intermittent streams are the predominant fluvial forms in arid and semi-arid environments, yet knowledge of how these streams function is limited; specifically, the linkages between wildlife and ecosystem characteristics in these environments are not well documented. Various studies have shown biological and habitat diversity in arid and semi-arid lands to be considerably higher along ephemeral and intermittent stream corridors in comparison to adjacent uplands. The central premise of this research is that the ecological integrity of these streams and the species that depend on them (including threatened, endangered, and at-risk species; TER-S) is inextricably linked to their hydrologic regime and that managing these systems is best addressed within an ecohydrological framework. The objective of this project was to develop an ecohydrologically-based stream type classification and methodology for four military installations in the southwestern United States representing the four Level III ecoregions that occur here: Fort Irwin (Mojave Basin and Range), Yuma Proving Ground (YPG; Sonoran Desert), Fort Huachuca (Madrean Archipelago), and Fort Bliss (Chihuahuan Desert). The goals of this project were to: (1) characterize ephemeral and intermittent streams using vegetation, hydrologic, and geomorphic attributes and develop a stream type classification based on those attributes, (2) associate wildlife habitat with the variables and stream type classification to improve management of TER-S and species of concern, and (3) provide a tool that allows Department of Defense (DoD) managers to evaluate the impacts of perturbations (e.g., climate change, military activities) on the hydrologic regimes of these systems and the species that depend on them.
Existing climatic, physical, and biological data from GIS and ground-based methods were acquired for each military installation to characterize the stream channels across that installation. Digital elevation models and imagery (including multi-return LIDAR and high resolution multispectral imagery) were used to quantify riparian vegetation cover, density, and structure. The Automated Geospatial Watershed Assessment Tool (AGWA) and its two embedded hydrologic models (Soil and Water Assessment Tool, SWAT and the Kinematic Runoff and Erosion Model, KINEROS2) were used to develop hydrologic gradients of peak flow and flow permanence. The U.S. Geological Survey (USGS) National Hydrography Plus Version 2 Dataset was used as the basis for the stream network and to derive the variables. Agglomerative hierarchical cluster analysis was used to classify ephemeral and intermittent streams by the ecohydrologic properties, and Classification and Regression Tree (CART) was used to determine thresholds for each variable for the predictive model. Various types of wildlife data, including species richness and occurrence of particular species, were analyzed in relation to the classified stream types and the ecohydrologic properties to provide information for managing species of concern. The AGWA tool was used to evaluate the impacts of climate change, training activities, and land management actions on flow permanence and peak flows.
Ephemeral and intermittent stream reaches at Fort Irwin, YPG, Fort Huachuca, and Fort Bliss were classified based on their hydrologic, geomorphic, and vegetation characteristics. Because these four installations have different hydrologic regimes and physical characteristics, each stream type classification is unique and represents the variability of stream reaches within that installation. The classification procedure was developed and applied using data specific to each installation and the data and methodologies are appropriate for use in extending the classification to streams not on the National Hydrography Dataset (NHD) stream network within each installation. The final ecohydrologic stream types were determined for each installation from statistical analyses, cluster validity tests, examination of the mapped clusters, and site knowledge. Fort Irwin, Fort Huachuca, and Fort Bliss had 8 final stream types, and YPG had 10 stream types. A CART classification tree was used to evaluate the clustering results for each installation to identify the thresholds for the input variables for each stream type. Results generally were explained in terms of the climate regime and geomorphology for YPG and Fort Irwin where annual rainfall amounts are relatively low or are largely confined to one season per year. Fort Bliss and Fort Huachuca experience higher annual rainfall amounts with a bimodal pattern, and vegetation variables were more important in those classifications. Vegetation density and cover were strongly related to elevation at all installations. The AGWA tool can be used to evaluate management actions or climate change scenarios that modify the input variables, to determine if and how the stream type and wildlife associations might be affected. All variables and results are spatially referenced using a “Unique ID” and can be used immediately to evaluate individual stream reaches for management needs.
The stream type classifications and input variables were associated with wildlife in two ways. Species richness models were developed using habitat models for each installation. Species distribution models that evaluated the use of streams by single species of concern were developed that may guide management of and future surveys for those species. At Fort Bliss, a model was created for the New Mexico state threatened Gray Vireo (Vireo vicinor). At Fort Huachuca, models were created for the federally threatened Mexican Spotted Owl (Strix occidentalis lucida) and Screech Owls (Western, Megascops kennicottii and Whiskered, Megascops trichopsis). At Fort Irwin, models were created for the federally threatened Desert Tortoise (Gopherus agassizi) and the Western Burrowing Owl (Athene cunicularis).
To assist management decision-making, datasets and tutorials were developed for evaluation of various land cover, land use, and climate change scenarios in AGWA. Technology transfer workshops were conducted at the installations to train personnel in the use of AGWA and to present project datasets and results. Feedback from the workshops was positive, indicating participants’ interest in using the tools generated by the project.
The classifications and wildlife associations will assist natural resource managers by providing a method of identifying similar stream types, as well as stream reaches with similar characteristics, and for use in developing sampling schemes or surveys for wildlife and land use management. This research provides benefits to the DoD that include: (1) advancement in scientific understanding of how ephemeral and intermittent streams function, (2) a procedure for producing an ecohydrologic classification of ephemeral and intermittent streams to guide management activities and decision making and that can be applied over large areas using remotely sensed data and watershed modeling tools, (3) associations of the classification and variables with wildlife habitat for improved understanding of habitat requirements and management of species of concern and TER-S, and (4) a tool for assessing impacts of perturbations (e.g., climate change, military activities) on the hydrologic regimes and species that depend upon these streams on DoD installations in the southwestern United States.