Irrigation
Runtime
Calculator for Drip and Microsprinkler Systems
User's Guide for
RTcitrus
IS003 Quick Answer
R. L. Snyder, Biometeorology Specialist
Department of Land, Air and Water Resources
University of California
Davis, CA 95616, USA
N.V. O'Connell, Farm Advisor
UCCE Tulare County
Ag Bldg Co Civic Ctr
Visalia, CA 93291, USA
Copyright
 Regents of the University of California
Created  July 2000
Last Revision –July 2001
Introduction
RTcitrus is an Excel program for calculating
an
irrigation schedule for citrus orchards using drip and microsprinkler
irrigation systems. To obtain a copy of the
program, click on RTcitrus.
To use the program,
you must know the mean application rate of the system in gallons per
hour per
emitter, the number of emitters per acre, and the distribution
uniformity. This
information is found by performing an irrigation system evaluation
using the
program "DU". The mean daily
reference evapotranspiration (ETo) rates and mean number of rainy days
by month
for your region are available from your local Cooperative Extension
Office, the
University of California Integrated Pest Management IPM web page, or from the California Irrigation
Management Information System (CIMIS).
Estimating
Crop Evapotranspiration
The RTcitrus program uses historical average
reference evapotranspiration (ET_{O})
rates and rainy days per month to compute daily ET_{O}
for bare soil over the year. The crop
coefficient (K_{C}) for bare soil is estimated
using equations that give
the K_{C} curves for various
wetting frequencies and ET_{O}
rates as shown in the figure below. The crop evapotranspiration (ET_{C}) rate for bare soil is
calculated using Equation 1.
_{}
(1)
The program also uses Equation 1, the ET_{O }rates and a peak K_{C}
value to estimate the crop
evapotranspiration for mature citrus over the year.
In general, the peak K_{C}
values for mature citrus are about 0.60, 0.65 and 0.70 for the desert,
Central
Valley, and coastal areas of California, respectively.
The correction factor (F) is used to estimate ET_{C}
for immature trees. For trees with less
than 63% ground cover (G_{C}),
_{}
(2)
and F =
1.0 for G_{C}³63%. Therefore,
the ET_{C} for a citrus orchard is calculated
as
_{}
(3)
Finally, for each day of the year, ET_{C} is calculated using
Equation (3) unless the calculated soil evaporation is higher. Then the soil evaporation rate is used for ET_{C}.
Calculating
Applied Water (Gross Application) Amounts
The actual amount to apply depends on the
application efficiency (AE) in
addition to the ET_{C}. Cumulative
ET_{C} between irrigation events provides an
estimate the
soil water depletion (SWD). Application
efficiency is the ratio of water
stored in the root zone for use in evapotranspiration divided by the
amount
applied. Assuming there is good
drainage, the AE is approximately equal to the system distribution
uniformity
(DU). Therefore, for welldrained
soils, the gross application (GA) is estimated as
_{}
where DU is the distribution uniformity
expressed as
a fraction.
Calculating
the Runtime
The runtime is computed by dividing the GA in
inches
by the application rate (AR) in inches per hour.
_{}
Using the
RTcitrus Program
The RTcitrus program has three input and two
output
worksheets. The historical _{} and rainfall
frequency data are entered by month into the "HETo" worksheet. Then the crop and irrigation system
information is input into the "System" worksheet. A
sample entry is shown in the figure
below. It is important to enter the
proper peak K_{C} value. The
year is input so the program knows whether to include February 29. The ET_{O}
zone is the zone where from the California ET_{O}
map where your orchard is located. For
immature orchards, estimated ground cover percentages must be input
next to the
proper date in the “System” worksheet.
If the cells are left blank, the program assumes that the crop
is
mature. The distribution uniformity,
application rate in gallons per hour per emitter and the number of
emitters per
acre must be input. The application
rates in gallons per minute per acre and in inches per hours will
automatically
appear.
Peak Kc = 
0.67 


Year = 
2001 


ETo Zone = 
12 






ground cover
on 1 Jan = 
70 
% 

ground cover
on 1 Apr = 
70 
% 

ground cover
on 1 Jul = 
70 
% 

ground cover
on 1 Oct = 
70 
% 

ground cover
on 31 Dec = 
70 
% 





Distribution
Uniformity = 
83 
% 

Application
Rate = 
6.01 
gph/emitter 

Number of
Emitters = 
121 
emitters/acre 













AR = 
12.1 
gpm/acre 

AR = 
0.027 
in/hr 
After input of the necessary
data, two output tables are computed.
The runtime needed to replace soil water loss to ET_{C}
is displayed in the worksheet "RT" by
month and day. In the worksheet
"CRT", the cumulative runtime is displayed for each day of the
year. These runtimes assume that there
is no contribution to ET_{C}
coming from rainfall. For growers with
a cover crop, a correction for the Kc is input directly into the
“Schedule”
worksheet (column H) on appropriate days. The correction should be a
fraction
(typically 0.250.35). The input value
is added to the Kc value of an orchard with no cover crop.
The peak Kc is not allowed to exceed
Kc=1.10.
To use the RT output table,
simply add up the number of hours from the previous to the new
irrigation
date. This is number of hours that your
system should be run to add approximately the cumulative ET_{C}
to the low quarter (i.e., the fourth of the field
receiving the least water). This means
that 3/4 of the crop will likely receive more than the SWD. However, using this method in a
welldrained field insures that most of the field is returned to field
capacity.
To use the CRT output table, subtract the
number of
hours on the previous from the new irrigation date.
This again is the number of hours that your system should be run
for a welldrained soil.
Soils with
Poor Drainage
Soils with poor drainage present a problem because the AE is bigger than the DU. There is no easy way to know AE in this situation. If the soil has a perched water table, it may be that all of the water applied is stored and can contribute to ET_{C}. Then, the AE would equal 100%. However, the water may not be distributed evenly across the orchard. However, if water logging is a problem, it may be beneficial to divide the SWD by a number bigger than the DU and possibly as big as 100%. This will result in a smaller application and less water logging. However, when you do this, be sure to monitor the water table depth at several locations in the orchard with a piezometer. To use the RTcitrus program for scheduling irrigation in an orchard with a perched water table, start with AE=DU and gradually increase the value for AE up to 100%. This should gradually decrease water logging in much of the orchard. If a water table still exists, increasing the AE to more than 100% may be necessary.