Irrigation Scheduling: A Guide for Efficient On-Farm Water Management
Contents
Introduction
Characterizing the Soil Environment
- Soil Water-Holding Properties
- Soil Texture
Soil-Based Monitoring
- Soil Moisture Sampling
- Tensiometers
- Gypsum Blocks
- Neutron Probes
- Thermal Dissipation Sensors
Plant-Based Monitoring
- Visible Sumptoms
- Pressure Bombs
- Infrared Thermometers
The Water Budget Approach
- Reference Evapotranspiration
- Effective Rainfall
- Crop Coefficients
- Yield Threshold Soil Water Depletion
- Scheduling Irrigation
Irrigation Efficiencies
- Efficiency and Uniformity
- Field Checks
Additional Considerations
- Shallow Water Tables
- Salinity
Implementing an Irrigation Strategy on the Farm
Appendix A: Crop Coefficients
Appendix B: Reference Crop Evapotranspiration
References and Bibliography
Introduction
Agriculture accounts for 85 percent of California's water usage. Increasing competition for the state's water supply from cities and industry, poor prospects for development of significant additional water supplies, and concern about the irrigation-related degradation of surface and groundwater quality have led to increasing emphasis on agricultural water conservation. Agriculture must get the most benefit from each unit of water for sustained crop productivity, and must at the same time maintain the quality of the state's land and water resources. Irrigation scheduling plays an important role in meeting these objectives.
In its simplest terms, irrigation scheduling means deciding when to irrgate and how much water to apply. The goal of an irrigation scheduling program is to supply the plants with adequate water while minimizing the loss of applied water - mainly to deep percolation and runoff. Irrigation scheduling depends upon various soil, atmospheric, crop, and irrigation system and operational factors. As such, no scheduling method is universally applicable. While researchers develop and evaluate different scheduling methods, the best irrigation management program from the grower's point of view is the one that is most profitable.
Some form of irrigation scheduling is practiced by every grower. However, the bases for making irrigation decisions and the levels of sophistication vary widely. They range from irrigation based on experience or on the practices of neighboring growers to techniques based on expensive, computer-aided instruments that assess soil, water or atmospheric paramters. These more recent methods generally can be grouped into two categories: (1) monitoring water status in the soil or plant, and (2) estimating crop water use based on atmospheric measurements, sometimes referred to as the water budget approach. In the past, atmospheric-based irrigation management has been hampered by the joint lack of economical, reliable measurement techniques and of information on water use rates for specific crops. Today, both are available. (For the former, see Chapters 2 and 3; for the latter, see Appendix A.)
Irrigation Scheduling is a book for growers and irrigation professionals. It can also be used as a teaching and technical resource. Its purpose is to provide information on various irrigation scheduling techniques in current use or under evaluation for future use. The publication provides a basic understanding of the soil, water, plant, atmospheric, and operational factors involved in scientific irrigation scheduling, as well as practical considerations in establishing an on-farm water management program. We have attempted to summarize the information, with emphasis on the operational aspects of each method, including strengths and weaknesses. Technical discussions of the methods are limited. The References and Bibliography section lists additional, more technical literature. We have also included chapters that address issues related to irrigation scheduling, including the texture, bulk density, field capacity, and available water of the soil (Chapter 1); as well as shallow water tables and salinity (Chapter 6); and a comprehensive list of historically averaged evapotranspiration figures for 208 California locations (Appendix B).
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