Modified Blaney-Criddle for Excel

PART B
WATER USE MANAGEMENT SYSTEM
DESIGN AND EVALUATION AIDS
I. SUPPLEMENTAL CROP REQUIREMENT
AND
WITHDRAWAL CALCULATION
SUPPLEMENTAL CROP REQUIREMENT AND WITHDRAVAL CALCULATION
Introduction
An important aspect of successful crop production or landscape maintenance is ensuring that the vegetation receives
sufficient moisture to meet its evapotranspiration demands. When rainfall is less than evapotranspiration, irrigation
must be applied to meet the deficit. As part of its consumptive water use permitting program, the South Florida Wate
Management District issues permits for irrigation water use. The goal of the permitting program is to provide an
allocation of irrigation water sufficient to meet the deficit between effective rainfall and evapotranspiration through a
drought event that has a probability of occurring once in every ten years (1-in-10). That allocation is designed to mee
both the needs of a prolonged drought of up to one year as well as to meet peak monthly demands
The South Florida Water Management District uses a modified Blaney-Criddle equation to determine crop
evapotranspiration and the method developed by the United States Department of Agriculture Soil Conservation
Service to determine supplemental irrigation needs
The Blaney-Criddle Equation
The Blaney-Criddle equation for estimating evapotransiration is
(1)
(1)
m
U = p t /100
1
where:
U = crop evapotranspiration for a given period,
k = an annual seasonal, or monthly empirical consumptive use coefficient which varies according to the cr
p = percent of daytime hours of the year which occur during the period,
t = mean temperature for the period, in Farenheit, and
m = month.
The equation has been modified to include:
(2) k = kt x kc
where:
kt = a climatic coefficient which is related to the mean air