7 Del. Admin. Code § 7103-91.0
Current through Register Vol. 28, No. 3, September 1, 2024
Section 7103-91.0 - Water Balance Data91.1 General91.1.1 Presented in the following sections are the Thornthwaite potential evapotranspiration calculation procedures and monthly mean temperatures, potential evapotranspiration, precipitation and 5-year return period precipitation for each of the long-term U.S. Weather Service stations in Delaware.91.2 Thornthwaite Potential Evapotranspiration91.2.1 The Thornthwaite Potential Evapotranspiration (P.E.T.) is defined as "the amount of water which will be lost from a surface completely covered with vegetation if there is sufficient water in the soil at all times for use of the vegetation." The Thornthwaite method is an empirical equation developed from correlations of mean monthly air temperature with evapotranspiration from water balance studies in valleys of the east-central United States where soil moisture conditions do not limit evapotranspiration. (The Irrigation Association, 1983, pp 112 to 114).91.3 Calculational Procedures91.3.1 The potential evapotranspiration is given by Thornthwaite as: 91.3.1.1 P.E.T. = 1.6 L (10 x Ta)/I)m in cm per month eq. 702-1 where I = 12 3i = 12 3(Ta/5)1.514 1 1
91.3.1.2 and where L = a daylength and month-length factor given in Table 702-1
Ta = the actual Celsius mean air temperature for the month
Ta = the climatological normal temperature in degrees
Celsius for each of the twelve months, and
m = a cubic function of the denominator of the term inside the bracket (the heat index I).
m = 6.75x10-7I3 - 7.71x10-5I2 + 1.79x102I + 0.449
91.3.1.3 The climatological normal mean is the 30-year mean updated every 10 years. Convert U.S. Weather Station temperatures in degrees Farenheit to degrees Celcius by the following: oC = (oF - 32) x 5/9
7 Del. Admin. Code § 7103-91.0