Rice field inventory using AVHRR data
D. Bachelet
Mantech Environmental Technology Inc.
US EPA Environmental Research Laboratory, Corvallis, Oregon, USA
J.L.Mailander
Department of Geosciences, Oregon State University, Corvallis, Oregon, USA
Abstract
Time series Normalized Difference Vegetation Index (NDVI) data, computed from Advanced Very high Resolution Radiometer (AVHRR) data, were used in pilot study to locate area for rice cultivation in the United States of America (USA). The large size of rice fields and the relative phonological homogeneity of the rice growing regions in the US make them ideal sites for a pilot study. NDVI dynamics were examined using 16 km global are coverage satellite data from 1988. Unsupervised classification was used to distinguish rice fields form other vegetation cover types. The technique was used for California where the contrast between irrigated and natural vegetation is the most pronounced and later applied to Louisiana, Arkansas and Texas. Identical methods were used to classify the vegetation in China where the field size is much smaller and the cropping season more extended. The rice NDVI dynamics was most obvious where only one crop is grown and the growing season is limited by low winter temperatures. Areas where several crops are grown each year were more difficult to identify. The effectively assess rice locations, the seasonal fluctuations of the crop, which are only partially dependent on seasonal precipitation because of irrigation, must be isolated from characteristics associated with natural vegetation and other irrigated corps.
Introduction
Global and regional scale ecological research has so far relied upon simple interpretations of land cover and surface properties and coarse resolution global land cover databases (Matthews 1983, Olson and Watts 1982, Wilson and Henderson Sellers 1985). Higher resolution data with greater precision for classification would clearly improve the quality of the research (IGBP 1991, Loveland et al 1991). During the last decade, substantial progress has been made in using National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) data for land cover characterization (Goward et. 1985, Tucker et al. 1986, Roller and Colwell 1986, Townshend
et. al 1991, Lloyd 1991, Loveland et al 1991). AVHRR data have moderate spatial resolution (1 km) but excellent temporal resolution (twice a day) which increase the likelihood of finding cloud-free observations during specific periods and makes it possible to monitor changes inland cover over periods as short as a growing season (Kennedy 1989, Johnson et al. 1987, Malingreu 1986. Justice et. al. 1985, Goward et al. 1985). Applications of AVHRR data have focused on the analysis of vegetation greenness which is most often characterized using a vegetation index such as the normalized difference vegetation index (NDVI) (Goward et al. 1985). Because of limitations in data availability, regional and global scale analyses have used resampled data in which either 4 or 16 km pixels aer combined (Global Vegetation Index or GVI). High Resolution (1 km) data are readily available for the USA but not yet available for Asia.
Rice is unique among grain crops in that it is generally flooded for a length of time at the beginning of the growing season. The NDVI signal should thus be low during flooding conditions, and much greater as the rice plant emerges from the standing water and eventually reaches peak vegetative growth. Malingreau (1986) plotted GVI signature of wetland rice in Asia using time series (1982-1985) AVHRR imagery. His results showed the applicability of coarse –scale imagery to describe the dynamics of an agricultural landscape. The objective of this paper was to determine the adequacy of AVHRR to identify rice growing regions at large scale. We used low resolution AVHRR data to identify rice growing areas in California and the Gulf States of the united Stated where locations and timing of the rice cultures are well known. We then attempted to extend the technique to the rice growing regions in the Hunan and Yangtze-delta of China where we had only few indications of the exact location of rice paddies.
