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Southeastern Geographer Vol. 21, No. 1, May 1981, pp. 26-39 COMPARATIVE CLIMATE-CORN YIELD RELATIONSHIPS IN THREE DELMARVA COUNTIES* Laurence S. Kalkstein and William C. Liebhardt Many attempts at predicting crop yields as a function of climate have been published recently, employing a variety of approaches. The most successful models incorporate water budget parameters such as évapotranspiration , moisture surplus, and moisture deficit rather than raw climatic data, as they are more precise in expressing true energy-moisture exchanges between the surface and the atmosphere. (J ) The use of daily, rather than mean monthly, climatic data represents a further refinement, since short-term fluctuations are de-emphasized or often eliminated when employing long-term means. (2) In addition, these predictive models most frequently employ multiple regression analysis, which is capable of handling a sizable number of independent climatic variables (assuming their orthogonality), and correlating them with the dependent variable, crop yield. (3) The proliferation of predictive models continues, but there are strikingly few attempts to analyze the results in terms of the causal climatic mechanisms. The timing and magnitude of a climatic event, as well as potential synergistic relationships among events, are all significant mechanisms, although their precise effects on yield are inadequately understood. In addition, the effect of the same external climatic conditions on a given crop may vary between regions because of edaphic influences. For example, trees of a given species have been shown to respond very differently to similar climatic stresses applied in nearby areas. (4) It is important to study annual crops as well to determine if they also demonstrate this trait. This paper will compare the effect of selected active factors of cli- * The authors wish to express their appreciation to Mr. Daniel J. Lawlor, Evapotranspiration Laboratory, Kansas State University, for his assistance in this study, and to extend their thanks to Ms. Anne Webster, Department of Geography , University of Delaware, for her cartographic assistance. Dr. Kalkstein is Assistant Professor of Geography at the Center for Climatic Research, University of Delaware, in Newark, DE 19711. Dr. Liebhardt is Associate Professor of Plant Science at the University of Delaware in Newark, DE 19711. Vol. XXI, No. 1 27 mate (potential évapotranspiration, water surplus, and water deficit) on corn yield in three counties on the Delmarva Peninsula. Intercounty similarities and differences in yield response to these factors will be isolated and evaluated. MATERIALS AND METHODS. Data on corn yield per acre were collected from state crop reporting agencies for the three evaluated counties : New Castle and Sussex in Delaware and Worcester in Maryland (Fig. 1). All three counties exhibit a similar warm, humid climate during the growing season so that intercounty differences in crop growth and yield are primarily attributed to factors influenced by edaphic differences , particularly moisture surplus and deficit. Variations in these parameters are especially sensitive to soil textural differences. For example , given a certain climatic situation, sandy coarse-grained soils exhibit larger surpluses and deficits than fine-grained loams and clays. (5) This is attributed to the smaller soil moisture storage capacities indicative of sandy soils, which inflates surplus values as field capacity is achieved more rapidly, and overland flow and groundwater supplies are increased correspondingly. In addition, deficit is augmented during dry periods, as less stored moisture is available to meet crop demands. (6) Soils of these three counties are classified as primarily Coastal Plain; however, they are quite variable. Soils of New Castle County are primarily silt loams (Ultisols), while Sussex County soils are mostly loamy sands and sandy loams (Entisols). Worcester County soils exhibit intermediate characteristics, since about 60 percent of the soils are silt loams and the remainder are loamy sands and sandy loams. For the purposes of this study, a rooting depth of 61 cm (2 feet) was assumed in making soil moisture storage calculations. Rooting depths are frequently deeper than 61 cm, but field observations on Delmarva indicate that the bulk of the roots is in the top 30 cm. Root volume at a depth exceeding 61 cm is minimal. Plow pans and natural pans which occur throughout much of Delmarva are primarily responsible for inhibiting rooting depth. These hard or cemented lenses are usually found at 30...


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