Efficient and precise irrigation management is critical if California producers are to maximize crop quality, conserve water, and protect the environment. The use of evapotranspiration (ET) estimates is a significant component of irrigation management. ET refers to the sum of water lost from the soil (evaporation) as well as that used by the crop (transpiration). While the California Irrigation Management Information System (CIMIS) network of weather stations derive daily ET values, there is a perception that CIMIS does not produce accurate ET estimates for all locations. This view is particularly prevalent in the canyons of Ventura County where weather conditions differ substantially compared to CIMIS locations. Since avocado and citrus thrive in these areas, it was concerning when it was determined that ET scheduling is not widely used.

That is, a Ventura County Resource Conservation District (RCD) review of California Department of Food and Agricultural State Water Efficiency and Enhancement Program (CDFA SWEEP) projects concluded that Ventura County growers substantially lagged their state-wide peers with respect to implementing ET-based irrigation scheduling (14% versus 44%).

RCD seeks to reverse the low implementation of ET-based irrigation scheduling within Ventura County by using simple, rugged on-site ET devices (atmometers) to determine on-site ET values. These on-site values will be compared to CIMIS values to determine local correction factors and develop refined ET maps for the canyon and valley areas. RCD will present these results at outreach events and provide workshops demonstrating how ET data, whether from CIMIS or on-site atmometers, can be used for irrigation management.

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PHOTO: Atmometer Test/Calibration Site @ UC Hansen

A recent review of how irrigation has been used world round has recently been published.  It's worth a read to learn how it hs been done and what we might learn for the presnet and the future. Daniele Zaccaria a UC Irrigation Specialist is one of the authors

Irrigation of World Agricultural Lands: Evolution through the Millennia

By: Andreas N. Angelakιs, Daniele Zaccaria, Jens Krasilnikoff, Miquel Salgot, Mohamed Bazza, Paolo Roccaro, Βlanca Jimenez, Arun Kumar, Wang Yinghua, Alper Baba, Jessica Anne Harrison, Andrea Garduno-Jimenezand Elias Fereres

(This article belongs to the Special Issue Water Supply and Water Scarcity)

Water 2020, 12(5), 1285; https://doi.org/10.3390/w12051285

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Abstract

Many agricultural production areas worldwide are characterized by high variability of water supply conditions, or simply lack of water, creating a dependence on irrigation since Neolithic times. The aim of this paper is to provide an overview of the evolution of irrigation of agricultural lands worldwide, based on bibliographical research focusing on ancient water management techniques and ingenious irrigation practices and their associated land management practices. In ancient Egypt, regular flooding by the Nile River meant that early agriculture probably consisted of planting seeds in soils that had been recently covered and fertilized with floodwater and silt deposits. On the other hand, in arid and semi-arid regions farmers made use of perennial springs and seasonal runoff under circumstances altogether different from the river civilizations of Mesopotamia, Egypt, India, and early dynasties in China. We review irrigation practices in all major irrigation regions through the centuries. Emphasis is given to the Bronze Age civilizations (Minoans, Egyptians, and Indus valley), pre-Columbian, civilizations from the historic times (e.g., Chinese, Hellenic, and Roman), late-Columbians (e.g., Aztecs and Incas) and Byzantines, as well as to Ottomans and Arabs. The implications and impacts of irrigation techniques on modern management of water resources, as well as on irrigated agriculture, are also considered and discussed. Finally, some current major agricultural water management challenges are outlined, concluding that ancient practices could be adapted to cope with present challenges in irrigated agriculture for increasing productivity and sustainability.

There's also some added discussion of drainage in this review paper

The Evolution of Agricultural Drainage

from the Earliest Times to the Present

Sustainability 2020, 12(1), 416;  https://www.mdpi.com/2071-1050/12/1/416/htm

Image: Egyptian Shaduf (from: Water Encyclopedia)

I just came across a wonderful summary of the different methods that can be used to schedule irrigations based on plant sensors.  In th past this is has been labor intensive and not easily adapted to commercial agriculture.  This review covers some of those problems and how they have or not been overcome by new technologies.  Of course, plant-based techniques are the not the only way to schedule and may only be part of the variety of methods used, such as soil-based ones like tensiometers; or weather-based ones, like ET.  A shovel and just looking at trees helps too.

Plant-Based Methods for Irrigation Scheduling

of Woody Crops

by José Enrique Fernández

ABSTRACT

The increasing world population and expected climate scenarios impel the agricultural sector towards a more efficient use of water. The scientific community is responding to that challenge by developing a variety of methods and technologies to increase crop water productivity. Precision irrigation is intended to achieve that purpose, through the wise choice of the irrigation system, the irrigation strategy, the method to schedule irrigation, and the production target. In this review, the relevance of precision irrigation for a rational use of water in agriculture, and methods related to the use of plant-based measurements for both the assessment of plant water stress and irrigation scheduling, are considered. These include non-automated, conventional methods based on manual records of plant water status and gas exchange, and automated methods where the related variable is recorded continuously and automatically. Thus, the use of methodologies based on the Scholander chamber and portable gas analysers, as well as those of systems for measuring sap flow, stem diameter variation and leaf turgor pressure, are reviewed. Other methods less used but with a potential to improve irrigation are also considered. These include those based on measurements related to the stem and leaf water content, and to changes in electrical potential within the plant. The use of measurements related to canopy temperature, both for direct assessment of water stress and for defining zones with different irrigation requirements, is also addressed. Finally, the importance of choosing the production target wisely, and the need for economic analyses to obtain maximum benefit of the technology related to precision irrigation, are outlined.

https://www.mdpi.com/2311-7524/3/2/35/htm

Every little bit helps, and the Ventura County Resource Conservation District might help some Ventura growers.

The Ventura County Resource Conservation District (VCRCD) would like to remind agricultural producers about an existing incentives program, the Calleguas Creek Watershed Agricultural Management Measures Program (CCWAMMP). The purpose of CCWAMMP is to improve water quality in the Frontal Pacific and Revolon Slough subwatersheds of Calleguas Creek. To achieve this, VCRCD, with funding from the State Water Resources Control Board, will reimburse growers a portion of the costs needed to implement certain agricultural management measures (MMs) and irrigation efficiency upgrades. If you are a grower in the coastal region of the Calleguas Creek Watershed, please submit an on-line CCWAMMP Interest Survey to VCRCD today! The CCWAMMP interest form is available here. 

VCRCD is also pleased to announce that a new incentives program, Interactive Irrigation Management to Reduce the Leaching of Nitrogen (MRLN), is expected to start April 2020. The goal of the MRLN program is to help agricultural producers build irrigation and fertilizer schedules that reduce the potential for nitrogen leaching. To achieve this goal, VCRCD will provide participants incentives for lysimeters and soil moisture sensing equipment as well as free irrigation and nutrient management technical assistance. Specifically, VCRCD will work with the landowner and agronomy professionals (such as Cooperative Extension staff) to evaluate the lysimeter and soil moisture data and provide the participating landowner guidance concerning potential irrigation and fertilization improvements. If you are near a nitrogen-impacted waterway in Ventura County, please submit a MRLN interest form here. 

For more information about either of these programs, please contact Jamie Whiteford at jamiewhiteford.vcrcd@gmail.com.