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Diverse irrigated areas were mapped in the Krishna River Basin (258,912 km 2 ), southern India, using an irrigated fraction approach and multiple ancillary data sources. Unsupervised classification of a monthly time series of net difference vegetation index (NDVI) images from the Moderate Resolution Imaging Spectrometer (MODIS) over January-December 2002 generated 40 classes. Nine generalized classes included five irrigated classes with distinct NDVI time signatures: continuous irrigation, double-cropped, irrigated with low biomass, minor irrigation, and groundwater irrigation. Areas irrigated by surface water began greening 45 days after groundwater-irrigated areas, which allowed separation of surface and groundwater irrigation in the classification. The fraction of each class area irrigated was determined using three different methods: ground truth data, a linear regression model calibrated to agricultural census data, and visual interpretation of Landsat TM imagery. Irrigated fractions determined by the three methods varied least for the double-cropped irrigated class (0.62-0.79) and rangeland (0.00-0.02), and most for the minor irrigated class (0.06-0.43). Small irrigated patches (,0.1 km 2 ) accounted for more irrigated area than all major surface water irrigated areas combined. The irrigated fractions of the minor and groundwater-irrigated classes differed widely by method, suggesting that mapping patchy and small irrigated areas remains challenging, but comparison of multiple data sources improves confidence in the classification and highlights areas requiring more intensive fieldwork.

Field-based soil moisture measurements are cumbersome. Remote sensing techniques based on active or passive microwave data have limitations. This paper presents and validates a new method based on land surface energy balances using remotely sensed optical data ͑including thermal infrared͒, which allows field and landscape-scale mapping of soil moisture depth-averaged through the root zone of existing vegetation. Root zone depth can be variable when crops are emerging. The pixel-wise ''evaporative fraction'' ͑ratio of latent heat flux to net available energy͒ is related to volumetric soil moisture through a standard regression curve that is independent of soil and vegetation type. Validation with measured root zone soil moisture in cropped soils in Mexico and Pakistan has a root mean square error of 0.05 cm 3 cm Ϫ3 ; the error is less than 0.07 cm 3 cm Ϫ3 in 90% of cases. Consequently, soil moisture data should be presented in class intervals of 0.05 cm 3 cm Ϫ3. The utility of this method is demonstrated at the field scale using multitemporal thematic mapper imagery for irrigated areas near Cortazar in Mexico, and for river basin-scale water resources distribution in Pakistan. The potential limitation is the presence of clouds and the time lag between consecutive images with field-scale resolution. With the falling price of optical satellite imagery, this technique should gain wider acceptance with river basin planners, watershed managers, and irrigation and drainage engineers.

[1] Three interlinked processes drive groundwater balances in diverse regions globally:(1) groundwater-irrigation intensification, (2) electrical energy supply for agriculture, and (3) climatic variability. Mexico's water-energy-climate nexus offers generic lessons because of its water scarcity and institutional reforms followed in other emerging economies. This paper analyzes data for 280 aquifers in Mexico, all registered water users, population projections, 2010-2100 precipitation and temperature projections for A1B and A2 emissions scenarios from 15 general circulation models, and 1999-2009 agricultural electricity use. Under A2 emissions, aquifers with negative balances will increase from 92 to 130 in number between 2010 and 2100, and the national groundwater deficit will increase by 21.3 km 3 . Under A2 and medium-variant population growth (which peaks midcentury), negative-balance aquifers will increase from 92 to 133, and the national groundwater deficit will increase by 22.4 km 3 . Agricultural power pricing offers a nexus-based policy tool to address aquifer depletion, an opportunity that was lost with the 2003 reduction in nighttime tariffs. Under A2, medium-variant population, and simulated 2% real annual increases in agricultural power tariffs, negative-balance aquifers will increase from 92 to 111, and the national groundwater deficit will increase by 17.5 km 3 between 2010 and 2100. Regulatory and user-based groundwater management initiatives indicate growing awareness of aquifer depletion; however, the long-term outlook points to continued depletion. This raises the need to harness nexus-based policy options, i.e., increasing agricultural power tariffs, eliminating reduced nighttime tariffs, enforcing legislation linking groundwater extraction to power use, and limiting new power connections for groundwater wells.

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