The New Mexico

Office of the State Engineer and the Interstate Stream Commission (OSE/ISC) are developing an Active Water Resource Management (AWRM) program for all basins within the state of New Mexico (see page 14). The AWRM will help New Mexico manage its water supply while complying with interstate compacts. At the same time, a technique for estimating irrigated acreages within each basin is being developed using satellite imagery and remote sensing technology. This approach is the first step in developing an annual water depletion model of the state.

The technique uses Landsat ETM+ (Enhancd Thematic Mapper) multispectral satellite imagery, which offers a number of advantages. The imagery is acquired from a stable sensor platform and therefore can easily be geometrically corrected to a base map. The height of the sensor above the earth (approximately 440 miles) negates most parallax problems commonly found in aerial photography. The satellite acquires imagery along a roughly north-south orbital path, recording data along an approximately 115-mile wide swath from west to east. This provides the large areal coverage needed for a regional study. In addition, the satellite revisits this path every 16 days, which facilitates the multi-temporal nature of this study. From a cost-analysis basis, Landsat data are cheaper than other higher spatial resolution data.

Before the data are ready for analysis, spatial and spectral processes are applied. The spatial process consists of rectifying the images to a map base, which defines the projection and coordinate system and allows area

26 • May/May/May June 2005 • Southwest Hydrology

measurements to be performed. The spectral process involves radiometric correction of the data to remove systematic

signal distortion of the sensor.

With the images rectified and radiometrically corrected, a digital analysis is possible. This analysis takes

advantage of the spectral characteristics of bands 3 and 4. Band 3 detects the absorption

of solar radiation by the active chlorophyll in green vegetation, and band 4 detects the

reflectance of chlorophyll. The ratio of the difference of these two bands, known as

the normalized difference vegetation index (NDVI), has been found to

correlate with the presence of green, healthy vegetation.

Each of the seasonal NDVI images are then mosaicked

together, and the mosaics are analyzed over known irrigated fields to find the NDVI value that

provides the best threshold separating the fields from

surrounding vegetation. Every value above the threshold

Dario Rodriguez-Bejarano, Ph.D. and Nabil Shafike, Ph.D. – Office of the State Engineer, New MexicoDario Rodriguez-Bejarano, Ph.D. and Nabil Shafike, Ph.D. – Office of the State Engineer, New Mexico

Remote Sensing to Estimate Irrigated Acreage in New Mexico

Composite NDVI image for the Lower Rio Grande Basin.

May/May/May June 2005 • Southwest Hydrology • 27

value is considered irrigated and changed to an output value of “1”; all values below the threshold are considered nonirrigated and changed to an output value of “0”. Each threshold seasonal mosaic then is reassigned such that spring images with a value of “1” retain a value of “1,” summer images of “1” are assigned a value of “3.” and fall images of “1” get a value of “5.” With these values reassigned, the seasonal mosaics are combined in a single image in which every possible combination of seasonal vegetative growth is assigned a unique value (see table above), and pixels are counted no more than once.

This method was applied to 2002 satellite imagery to estimate the irrigated land in the Lower Rio Grande (LRG) Basin in New Mexico. The LRG was extracted from a Rio Grande sub-area and refined further to show the western, eastern and North Las Cruces portions of the LRG from Caballo Dam to the border with Texas. The NDVI results of these three areas as well as the corridor along the Rio Grande then were clipped from the statewide NDVI image and the irrigated

area was computed. The table at right shows the resulting estimation compared to the New Mexico Office of the State Engineer’s Hydrographic Survey Bureau (HSB) estimate; the estimates for the Nutt-Hockett Basin are also shown.

The results show that the NDVI methodology provides an adequate estimate for irrigated lands and consumptive use of water statewide. Although the approach calls for using Landsat ETM+ imagery, which will not be available in the future, past experience both within the OSE/ISC and by out-of-state agencies indicate that higher spatial resolution satellite systems, digital low-flying aircraft imagery, or aerial photography may be suitable substitutes.

OSE/ISC has identified and is using advanced technologies to produce a GIS data layer of the entire state that contains all relevant details of a given water

right. As part of this process, OSE/ISC has begun acquiring multispectral airborne digital imagery of high spatial resolution to replace its aerial photography. The objective behind obtaining digital airborne imagery is to replace past aerial photography with a product that has greater spectral resolution, similar spatial resolution, up-front computer compatibility, and the capability to simultaneously use multiple-image analyses.

The authors wish to acknowledge the contribution and technical work of Paul Neville of the Earth Data Analysis Center (EDAC) at the University of New Mexico. Contact Dario Rodriguez-Bejarano at drodriguez@ose.state.nm.us or Nabil Shafike at nshafike@ose.state.nm.us.

Remote Sensing to Estimate Irrigated Acreage in New Mexico

Vegetative Growth Status ValueNo Vegetation 0Spring Only 1Summer Only 3Fall Only 5Spring and Summer Only 4Summer and Fall Only 8Spring and Fall Only 6All Seasons 9

AreaHSB

Acreage Estimate

NDVI AcreageEstimate

Nutt-Hockett 11,554 10,971LRG* 99,182 95,888Totals 110,736 106,859

* Includes Rincon, Northern Mesilla, Southern Mesilla and outlying areas.

HSB cropland acreage and NDVI acreage estimates for 2002.

Value assigned based on vegetative growth status.