Comparison of methods for estimation of drought intensity and the relationship of drought to yields of agronomic crops in Tennessee

Delineation of the general effects of drought on crops in Tennessee and the critical periods during which crop yields are significantly affected had not been performed for Tennessee. Purposes of this study were:

1. To calculate drought-days using evapotranspiration values derived from four water-use models;

2. To evaluate each model by application to variety trial yields of corn, cotton, and burley tobacco varieties from several locations in the state;

3. To select the best model for use under Tennessee conditions and to calculate drought probabilities from those data; and

4. To evaluate county yield estimates in terms of their response to drought-days as calculated by the best model.

Evapotranspiration values were calculated using the methods described by Penman, Blaney-Criddle, and Thornthwaite; drought-days were calculated using these values and were summed by climatic weeks and by calendar months. An adjusted Thornthwaite method developed by Wiser and van Schilfgaarde which only calculated drought-days on a monthly basis was also evaluated.

Crop yield variation proved to be most sensitive to drought calculated using Thornthwaite-derived evapotranspiration values. Crop response to Blaney-Criddle-derived drought was intermediate, while the yield variation response to Penman-derived values indicated that the latter method was least sensitive to prevailing conditions. Drought totals per month as calculated by the Wiser modification as well as monthly totals from the Thornthwaite equation proved to be less sensitive to actual conditions as reflected in crop yield than did weekly totals of drought-days. This was primarily due to the masking of within-month variation of moisture.status which was more precisely revealed by weekly drought-day totals. Relative lack of predictive power of Penman-derived drought totals was attributed to the complexity of the formula and the lack of truly adequate data from many locations within Tennessee. Regressions of weekly drought-day totals against variety trial yields indicated that critical periods for corn growth occurred primarily during germination, during anthesis, and during grain-filling. Weekly periods critical for cotton yield were approximately the same for all West Tennessee locations; however, moisture stress was more significant at Fort Pillow and Ames Plantation than at Jackson or Knoxville. Tobacco yields varied with significant deficits occurring from the first of July through August, In general, for all crops, stress at almost any stage between floral initiation and maturity was likely to result in yield variation. At those general levels of available water within the effective rooting zone it appeared that the preflowering stage was the most tolerant to moisture deficits however, drought at flowering and during maturation were the primary causes of yield reduction and/or variation.

Probabilities of drought occurrence were calculated using Thornthwaite ET values for each of the 50 selected locations.

Analyses of county yield estimates indicated that moisture status does play a role in production; however, due to extreme linearity of these yield estimates and to the nature of their derivation no further interpretation was considered appropriate.