Soils 456 Notes Chapter 6

Irrigation scheduling must answer two questions when and how much to irrigate.

1. Infiltration
i= at^-0.5
I= at^b +c

where :
i= infiltration  rate inch/hr
I= accumulative infiltration inches
t=time hr
a,b,c constants

See Campbells model to simulate infiltration and resdistribution

Runoff occurs when the rainfall or irrigation rate exceeds the soil's infiltration capacity. On sloping areas, runoff is a concern since it can carry soil particles, nutrients, and other chemicals with it.

Darcy's equation controls the infiltration rate.

Saturated hydraulic conductivity if a function of soil type

2. Water Holding Capacity

Methods to measure soil water content. 

Available water= water/ft * rooting depth  DARCY’S Law and Saturated Flow:

Available water/ft soil= (water content at field capacity(volume) - water content at Permanent wilting point(volume))*1ft depth
Water content (volume) = water content (weight (g water/g soil)) * density of soil

Field Capacity is assumed to be the water content at 1/3 of a bar
Permanent wilting point is assumed to be the water content at 15 bars.


Soil water characteristic curve determine field capacity and permanent wilting point.  See calculator for conversions.   and see soil calculator

  General table for soil types

3. Plant water usage

Potential evapotranspiration acquired from climate data and varies by climate Actual evapotranspiration Et= Pet* crop coefficient. Actual evapotranspiration is measured using lysimeters and a crop coefficient calculated that is used in other locations. Simple lysimeter can be built with PVC pipe, usually 18 to 24 inch (45 to 60 cm) diameter, vertically placed, filled with soil and a flat cap glued to the bottom. Each lysimeter or PVC column is placed on a water pillow which has a tygon tube extended from the pillow. This tube is fixed vertically along and parallel to the PVC lysimeter packed with soil. The change in weight due to irrigations or ET can determine by seeing differences in the height of the tygon tube 'manometer' especially if you placed an inexpensive ruler behind the tube to note where the water level was each day (morning). There need to be 'dummy' lysimeters (capped on both ends filled with soil) within the assortment of lysmeters to record temperature anomolies of the pillowed water.

A first estimate of the crop coefficient can be determined by estimating the percent cover . If the percent cover is 20% then the crop coefficient is 0.2 Percent cover cans be estimated using a ruler and measuring the length of the ruler that sees plant material compared to the length of the ruler that sees bare soil.

Crop coefficients include evaporation from soil. See for a discussion on evaporation component. 

Irrigate when RAW is used up.   Results of tomato experiment 

RAW( readily available water)= AW( available water)* MAD( management allowed depletion ) see tensiometers plus example in book. . See Alfalfa water use under moisture stress.
Roots extract most water from the upper half of the root zone. Calculations of MAD are for the entire root zone.

Water budget slide show.

Calculation of water budget

4. Depth to apply

Net application depth= irrigation interval/ usage/day.

5. Efficiency.

A. Application Efficiency= water stored in root zone/ water applied to system 100%

B. Gross Application depth= net irrigation depth/ efficiency/100

To calculate the time to apply a given depth of water used qt=ad
C: Scheduling irrigation's using reference evapotranspiration and an irrigation controller

6. Spreed sheet for irrigation sheduling using the water budget approach for chile.

7. Stress Functions causing Et and yield to decrease to due to salinity streess, water stress, and nitrogen stress.

8. Leaching Requirenments and conversion units of salinity, See USDA salinity lab for salinity stress functions.

If you have any questions please contact
Updated: Jan 18 1998
Copyright 1996 New Mexico State University
Department of Agronomy and Horticulture
Box 30001 / Dept.3Q / Las Cruces, N M 88003-8003
Telephone: (505)646-3405
FAX: (505)646-6041