New Corn Evapotranspiration Crop Curves for Southeastern North Dakota

Accurate irrigation scheduling requires knowledge of crop water use. Crop curves are one means of irrigation scheduling. Crop curves are the ratio of actual crop water use to reference crop evapotranspiration presented as a function of an independent variable such as days past planting, phenological development, fraction of season, or cumulative seasonal heat units. Crop curves are commonly based on daily data from weighing lysimeters. In nonweighing lysimeters, soil moisture measurements may be taken at weekly or other nondaily intervals. We present four mean corn evapotranspiration crop curves based on 11 years of data from four nonweighing lysimeters. The crop curves are based on Jensen-Haise (J-H) or Penman-Allen (P-A) reference evapotranspiration (ETr) computations as functions of days past planting (DPP) or cumulative growing degree days (CGDD) since planting. The r2 and standard errors in the Kc estimates (SEKc) were 0.68 and 0.21, respectively, for the J-H DPP method; 0.54 and 0.25 for the J-H CGDD method; 0.62 and 0.24 for the P-A DPP method; and 0.52 and 0.27 for the P-A CGDD method. We also illustrate: 1) calculation procedures for development of crop curves based on nondaily soil moisture measurements; 2) a method of referencing each water balance period to the independent variable which does not bias the resulting crop curve forward or backward in time; and 3) the repercussions of using an insufficient number of significant digits in the crop curve polynomial coefficients. Reliable and unbiased crop curves should result from use of the methods described in this article.