Big Nut in New Mexico
Pecan production and water use information collected by satellite
By Leeann DeMouche
Pecan production in New Mexico is a big business, where annual cash receipts have been anywhere from $85 million to more than $100 million annually (U.S. Department of Agriculture-National Agricultural Statistics Service), causing a growing interest in increased pecan acreage. Acreage for pecan production in the 2002 USDA Census shows New Mexico at 38,000 acres, where 6 percent of that acreage is considered to be non-baring or open canopy. Researchers at New Mexico State University (NMSU) are working to help pecan growers of open-canopy orchards earn more money while protecting their trees from water stress.
Seasonally, pecan consumptive water use is high compared to most crops and is estimated at 52 inches for mature trees. Thus, farmers need to maximize irrigation water use efficiency. Consumptive water use of pecans is a combination of evaporation from the soil and transpiration through the leaves. Researchers in the Plant and Environmental Sciences Department at NMSU are using a camera attached to a 4-foot diameter remote control balloon to collect the area of pecan canopy. The canopy area is then calculated using Photoshop software that identifies all areas with a green cover. A second method to collectcanopy is to obtain high resolution satellite images taken by the Quickbird-2 satellite from Digital Globe Corporation and again use Photoshop to calculate the project cover. These two processes can occur simultaneously and are usually measured together as evapotranspiration (ET).
Weather conditions from weather stations can also be used to reference potential ET of a crop (well-watered grass or alfalfa), ETr. The ETr is then multiplied by a crop coefficient (Kc) scaling factor to obtain crop ET (ETc) under standard conditions. Standard conditions are defined as large fields under optimum soil water, excellent management and environmental conditions that allow the crop to achieve full production under the given climatic conditions.
The effect of tree age and spacing is manifested in the amount of canopy cover. Consequently, when pecan trees are young, the water use is proportional to the amount of solar radiation intercepted by the canopy. The water use at closed canopy (Et= Kc Etr) can bescaled to the water use of an open canopy by the effective canopy cover (ECC) where ECC is defined as the proportion of the soil surface shaded by a crop at solar noon.
Researchers at NMSU have found that measurements of the canopy cover from satellite or balloon data can assist in the calculation of and the proper amount of water needed for irrigation of a small orchard.
Researchers have observed that when an orchard has more than 78 percent cover, ET does not increase with increasing cover. Consequently, by combining many technologies together, irrigation scheduling for trees with incomplete cover is possible. The main problem is to get a realistic measurement of the canopy cover in midsummer at the time when the orchard has reach maximum cover.
