Lecture 2 - Irrigation Management

Rain-Fed Agriculture

  • Potentially extractive, reliant on natural rainfall.

  • Shouldn't alter local hydrology.

  • Accounts for approximately 60% of global crop production.

  • Example: North China Plain allows only one crop per year with rain-fed agriculture.

  • Population pressure leads to double cropping where water is available, often using "fossil water" from subterranean aquifers.

    • This extraction lowers water table heights.

    • Water is pumped to the surface for irrigation, enabling two crops per year to sustain production and population.

Irrigated Agriculture Controversies

  • A controversial area of water management.

  • Examples of challenges:

    • Australian cotton growers using excessive water (five times Sydney Harbour volume annually).

    • Rural communities lacking drinking water due to upstream irrigation; water trucked over long distances (e.g., 200 km).

    • Government spending millions to buy water rights from irrigators to improve ecological health.

    • Corruption scandals delaying irrigation scheme approvals (e.g., Peru, reliant on Andes snowmelt).

    • Viability questioned due to climate change and altered snow accumulation.

Key Relationships: Irrigation, Crop Water Use, and Yields

  • Understanding these relationships is key.

  • Resource use efficiency graphs help assess sustainability.

  • Economics crucial:

    • Water pricing and crop quality.

    • Farmer profits and attitudes to risk affect adoption of irrigation techniques.

    • Even if science demonstrates water-saving potential, farmers need convincing of water security.

Implications for Planetary Health

  • Rice cultivation:

    • High water demand, often grown in flooded soil.

    • Significant contributor to global greenhouse gas emissions, especially methane.

Yield vs. Evapotranspiration

  • Rain-fed agriculture: Linear relationship between yield and evapotranspiration.

    • Aim: Maximize water use while regulating it throughout the season.

    • Consider water supply demand functions that vary throughout the season.

Irrigated Agriculture: Yield vs. Applied Water

  • Curvilinear relationship.

    • Deficit irrigation: Applying less water than optimal.

    • Economic irrigation rate: Balancing water application costs with yield value.

    • Typical strategy: Irrigating to maximum yield, but often over-watering.

    • Excess water can limit crop growth and yield.

Focus of Irrigation Science

  • Identifying opportunities for water conservation.

  • Convincing farmers to shift from over-watering to more efficient practices.

  • Field-scale experiments:

    • Comparing farmer practices, scientifically scheduled optimal water supply, and deficit irrigation techniques.

    • Demonstrating that reduced water application can achieve desired crop quality and quantity, leading to changes in farmer behavior.

Sustained Deficit Irrigation

  • Applying less water throughout the crop lifecycle.

  • Determining optimal water volume for specific crops and locations to achieve specific yields.

Sophisticated Irrigation Techniques

  • Considering:

    • Timing of irrigation relative to crop development.

    • Placement of irrigation on the soil surface.

    • Frequency of irrigation.

Irrigation Techniques

  • RDI (Regulated Deficit Irrigation).

  • PRD (Partial Root Zone Drying).

  • AWD (Alternate Wetting and Drying).

Regulated Deficit Irrigation (RDI)

  • Suspending or reducing water application during less sensitive crop stages.

  • Examples:

    • Post-harvest.

    • Early grape berry development.

  • Timing based on crop development to minimize impact of water deficit.

Partial Root Zone Drying (PRD)

  • Varying water placement on root surface.

  • Concentrating water on one side of the crop row.

  • Variations:

    • Fixed PRD: Wet and dry sides remain constant.

    • Alternate PRD: Swapping wet and dry sides.

Alternate Wetting and Drying (AWD)

  • Specific to rice cultivation.

  • Alternating between flooding and drying the soil.

  • Method: Using a perforated plastic tube to monitor water table height.

    • Reflooding when water table drops to approximately 15 cm below the surface.

  • Benefits:

    • Significant water savings.

    • Reduced greenhouse gas emissions.

Farmer Practices and Water Productivity

  • Irrigated Water Productivity: Euros per cubic meter of water.

    • Different crops yield varying profits per water unit.

    • Water availability influences crop choice.

Diversity of Farm Irrigation Practices

  • Significant variation even within a single irrigation district with a single crop.

  • Farmers apply irrigation at different stages of the season.

  • Need to understand drivers and impacts of these diverse behaviors on crop yields.

Determining Crop Water Requirements

  • Essential to determine actual water needs for individual crops, regions, or irrigation districts. *Example: Southeast Spain (Murcia):

    • High productivity in horticulture.

    • Calculations show a deficit where cropping area exceeds sustainable irrigation supplies.

Available Irrigation Supplies

*Rainfall: Limited (e.g., 293 mm/year).
*Water pumped from central Spain.
*Aquifer water: Low quality, high salinity, potentially compromising sustainability.
*Investment in drip irrigation systems for increased efficiency.
*Use of regulated deficit irrigation.

Cost-Benefit Analysis of Irrigation Techniques

  • Study comparing irrigation techniques:
    Control: Applying consistent water throughout crop development (60% of evapotranspiration), considered sustained deficit irrigation.
    Deficit irrigation: Applying even less water during specific periods between fruit set and harvest.
    Recovery period: Allowing crop growth after harvest for bud development.
    *Biochemical changes: Focus on sugar and color accumulation in berries determining wine quality.
    *Net margin analysis.

Economic vs. Ecological Sustainability

  • Control treatment (more water) was more profitable due to higher yields, lack of payment premiums for high quality fruit (more berries = more profit).
    Deficit irrigation techniques only became profitable with increased grape prices.
    Control was always highly profitable.
    Water price increases needed to make deficit irrigation techniques worthwhile.
    Deficit irrigation techniques improve grape quality (color, biochemistry) but aren't currently economically incentivized.

Conclusions (Part 1)

  • Irrigation scheduling predominantly aims to determine volumes to maximize optimal crop yields.
    Playing with timing, placement, and frequency can impact yield.
    Deficit irrigation: Applying less water to save water and maintain yield, but requires economic incentives (increased water price, higher product value).

Partial Root Zone Drying (PRD) Origins

  • Invented at Lancaster, derived from plant physiology research.
    Question: Is stomatal conductance regulated by leaf water relations?
    Experiments exposed plants to both well-watered and dry soil conditions. Results: Plants exposed to dry soil maintained higher turgor due to stomatal closure, roots sense soil dryness, sending chemical signals (ABA) to control stomata.

ABA as a Root-to-Shoot Signal

  • Soil drying enhances ABA production in roots.

  • ABA is transported via xylem sap to shoots, closing stomata.

  • Relationship between water use efficiency and xylem ABA concentration.

  • Deficit irrigation techniques increase water use efficiency by closing stomata while maintaining photosynthesis.

Field Application of PRD in Australia

  • Split root system applied to grapevines with drippers on both sides of each row, irrigating one part of root system. alternating the wet and dry sides. significant reduction in water usage, with maintained fruit yield.

Challenges with PRD

  • Confounding of factors: Irrigation volume and placement are changed making analysis more difficult.
    PRD may not offer benefits over conventional irrigation. There may be nothing magic about alternating wet and dry sides.
    Experiments required to evaluate PRD value and alternate between wet and dry sides to maximize signal.

Root Biomass Distribution

  • Significant change in root biomass distribution with alternating wetting and drying.
    Significant boost by re-watering this previously dry part.
    If this root system is unrestricted this can access water deeper in the soil profile, thus important to the success of the partial root zone drying.

Current Research and Challenges

  • Increased interest in PRD due to water scarcity.
    Commercial applications: Strong links between farmers and universities or in extremely dry locations.
    Applied in high-tech arable horticulture/viticulture (relationships with universities) or low-tech arable agriculture (furrow irrigation).

Multiple Choice Question (Plant Water Relations)

  • Check for understanding of plant water relations required.

Application Challenges

  • PRD may be too challenging for some farmers to adopt.

Alternate Wetting and Drying (AWD) for Rice

  • Wide promotion for rice cultivation.
    Water is allowed to drop to about 15 centimeters below the soil surface. This is a simple, low cost, and farmer friendly as long as there are measurements of the water table height.
    Substaintial savings can be accomplished with no limiting yield, also reducing pumping cost.

Evaluation of Alternate Wetting and Drying (AWD)

  • Limited change in yield, but substantial water savings, substantial increase in water productivity.
    Under the precise circumstances, there can be a yield reduction. If there is a specific soil type or set of conditions with a yield penalty, it can lower the belief of using this technique.

Conclusion:

There options we can use to reduce water use, including options effecting the physiology and yield.
When in the correct economic environment, the policy and farmer education is there, it can lead to a positive impact.