Keywords: Wetland vegetation, NDVI, trends, patterns

Abstract
The purpose of this research was to characterize the patterns and trends of wetland vegetation for an area around Island Lake in the Sandhills of Nebraska, USA. Within the Sandhills there are numerous shallow lakes, and along their borders a significant amount of wetland vegetation growth has been observed. In this study the Normalized Difference Vegetation Index (NDVI) was used to examine its variation across different terrain features within the landscape. Landsat MSS data acquired between 1979 and 1989 during May-October (growing season) were used to derive NDVI values for selected 5x5 windows at specific locations. These windows represented a marsh area, wet meadow, sand dune slope, and sand dune top. NDVI trends over the 11-year period were determined and average NDVI values along with standard deviation were computed for each year. The highest mean NDVI was recorded for the marsh, while the lowest occurred on the dune top. Results also showed that the marsh was prone to higher variation in NDVI from year-to-year than any of the other terrain types.

Introduction
It has been well documented that wetlands are an important natural resource because they assimilate pollutants, provide flood control, and serve as breeding, nursery, and feeding grounds for a variety of fish and wildlife (Odum, 1989). Remote sensing offers a means by which wetland environments can be monitored and analyzed (Work and Gilmer, 1976; Jensen et al., 1991). In addition, through the use of historical aerial photography and satellite imagery (e.g., Landsat MSS, TM), it is possible to perform a long-term analysis of changes in wetland areas and evaluate any patterns or trends that may be observed. This research focuses on using a historical archive of Landsat Multispectral Scanner (MSS) data and analyzes the variations in the NDVI over the different terrain characteristics of the landscape.

Study Area
The study area for this project was Island Lake, which is located in the western portion of an area known as the Sandhills region of Nebraska in the U.S. (Figure 1). The Sandhills region covers approximately 57,000 km² and is comprised of stabilized sand dunes modified by deflation hollows, small shallow lakes, marshes, and subirrigated meadows (Rundquist et al., 1985). Lying practically in the center of the North American continent, the Sandhills fall in a semi-arid climatic zone. Precipitation in the area ranges from 63.5 cm in the east to 40 cm in the west, with very little of it resulting in surface runoff because of the sandy soil.


Figure 1. Location of Island Lake study area and the four topographic sites selected for analysis.
Methodology
A total of 57 Landsat MSS scenes (subset for Island Lake) from 1979-1989 were selected for this research (Table 1). All data were geometrically rectified to the Universal Transverse Mercator (UTM) projection, and registered to each other. Four areas representing various facets of the topography within the area surrounding Island Lake were selected. These represented

• marshland (i.e., wetland area);
• wet meadow;
• dune slope; and
• dune top.

The Normalized Difference Vegetation Index (NDVI) was computed for a 5x5 window representing each topographic type (see Figure 1). Seasonal patterns and trends, annual mean, and standard deviation of the NDVI were computed for all data sets.
Table 1. Dates of Landsat MSS imagery used to evaluate NDVI trends for Island Lake.

	1979	1980	1981	1982	1983	1984	1985	1986	1987	1988	1989
May			5/01 5/19		5/04	5/14 5/30		5/04	5/07		5/20
June	6/25	6/11 6/29	6/06 6/24	6/28	6/21	6/15	6/02 6/18
July		7/16	7/11 7/30	7/16	7/07	7/17	7/04		7/10 7/26		7/23
Aug	8/01	8/04 8/22			8/08	8/10			8/11	8/13 8/29	8/8
Sept	9/06 9/14 9/24 9/25	9/18 9/27			9/09	9/03 9/19		9/25	9/12 9/28		9/25
Oct	10/02	10/14	10/10 10/16	10/14	10/27		10/24