The Distribution of Early-winter Flooding in the Central Valley of California: 2000-2010

The Central Valley of California (CVC) is an important region for wintering shorebirds in the Pacific Flyway. Despite the importance of the CVC for Pacific Flyway shorebirds, currently there is no regular monitoring to quantify spatial and temporal variation and trends in shorebird populations using this landscape. Although the distribution shorebird habitat, including managed wetlands, vernal pools, and evaporation ponds, is mostly known and typically relatively stable over time, spatial and temporal variation in the distribution of flooded agricultural shorebird habitat is not well understood. Due to their ability to respond quickly to changing habitat conditions, shorebirds may shift their wintering distribution in response annual or even within season changes in the distribution of their habitat. In order to develop a robust monitoring plan for wintering shorebirds, it is important to understand the spatial and temporal distribution of their habitat during the chosen survey window. Remote sensing is a powerful tool to track habitat changes on a broad landscape and satellite based imagery is widely available. In this report we present: (1) GIS layers documenting the distribution of water and non-water areas during the early winter in the CVC between 2000 and 2010; (2) an aggregated GIS layer identifying the average probability of water presence for each pixel in the CVC; and (3) quantification of the spatial and temporal variability of water during early winter in the CVC. We acquired Landsat images of 3 scenes covering ~85% of the CVC and representing 10 winters (2000 – 2001 to 2009 – 2010). We classified image pixels into water and non-water. Overall, our classification summaries suggested that the total area of flooded habitat has been relatively stable through time in the CVC however there is significant year to year variation in the total amount of flooded habitat occurs in some basins. The Tulare Basin and the Delta Basin exhibited the largest year to year variation in flooded habitat. The largest extent of regular flooding (>30% of years) occurred in the north scene which largely represents flooded post-harvest rice as well as the extensive managed wetlands in this region. Overall, our approach was able to predict the spatial distribution of water in the CVC across many years using a simple classification technique. Our rapid assessment of water and non-water and evaluation of these data provided a broad spatial and temporal scale perspective on the distribution of surface water. These data provide needed habitat information for designing a robust monitoring program for wintering shorebirds in the CVC.