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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ ~ ~ ~ ~ ~ ~ . . . . I I I Aquifer. Annual precipitation in the area served by the High Plains UWCD's project was estimated 10 have produced '.47 to 1 .97 inches more in 1997 and 1999, respectively. than the 1945 through 1997 long-term naturally OCCtlrring average of 1829 inches. For the 20.294 SQuare mile (12.988.160-acre) Llano Estacado Planning Region. an increase in precipitation of 1.5 inch would result in an increase of about 1,623,520 acre-feel of water per year 10 the land surface. At a cost of 7.2 cents per acre, the cost per acre-feel of water is 50.57 (1WDB. 2002). Additional precipitation during the growing season. which is the period during which the Southern Ogallala doud seeding project is operated. would directly and immediately benefit dryland and irrigated agriculture. Crop and grazing yields could be increased. irrigation water pumped from the Ogallala Aquifer could be reduced, and lawn irrigation could be reduced. The latter effect would contribute to meeting projected municipal water needs by reducing the quantities used per year from present supplies. Additional rainfall runoff would be collected in public water supply surface water reservoi~ and in playa lakes, which could increase recharge to the aquifer, as well as provide water for wildlife. A climatology of the Southern Ogallala target area The climate of the llano Estacado Region is dassifled as a dry. steppe type. The region is characterized as semi"arid, with a wide range in temperatures. In an average year, about 80 percent of the annual rainfall OCCU~ during the warm season (May through October). Monthly rainfall quantities ordinarily dedine markedly in the colder months of the year, when frequent periods of cold, dry air from North American Polar Regions surge southward and cut off the supply of moisture from the Gutf of Mexico and the Mexican Monsoon. The long"term average (1945 through 1997) precipitation received in the region is 18.4 inches. The average ranges from a high of 22 inches per year in a small area in Crosby County, to a low of about 16 inches in Cochran County in the southwestern portion of the region. Most of this precipitation is mainly convective during the warm season and thus it is more localized and its spatial and temporal distribution as well as the intensity of rainfall is highly variable. One of the fundamental aspects of forecasting convection is the diagnosis of the large"scale thermodynamic environment In addition, the depth and magnitude of low"level moisture that fuels thunderstorms, and the thermodynamic characteristics of the over1ying air stream, taken in the context of potential triggering mechanisms and environmental dynamics are significant factors to be considered in any convective forecast. Early in the growing season. convective initiation is caused by eastward moving cyclones. The topography of the region channels warm moist air from the Gulf of Mexico northward and cold dry arctic air from Canada southward. These two very different air masses, together with wester1y downslope flow off the Rockies, have important influences on the convective mechanisms in the target area. The confluence of warm dry air off the southern Rockies and warm moist air from the Gulf of Mexico produces a strong west-east gradient in moisture called the dryline. The motion of the spring and summertime southern High Plains dryline in the absence of any large- scale weather systems usually exhibits a diurnal trend. In general. this trend is described as an easter1y advance of the dryline during the daytime and a westerly retreat at night. It is known that convective development often is collocated with the dryline and such development may lead to thunderstorm initiation dependent upon various airmass properties in the vicinity of the dryline. Another important feature during this period is the cold front and how it combines with the dryline to act as a focus for convective development. The middle of the growing season is characterized by a period of transition from the cold season circulation regime to the warm season regime. This is accompanied by a decrease in mid-latitude synoptic"scale transient actiVity over the continental United States as the extratropical stonn track weakens and migrates poleward to a position near the Canadian border. The upper air becomes rather stagnant with a persistent 7