Traditional textbooks and reference materials primarily focus on the planning and design stages of agricultural irrigation projects, detailing the requirements for design and construction. Consequently, when designing irrigation systems, one can easily find a wealth of relevant materials, manuals, and even textbooks for reference. However, compared to design, there are relatively fewer reference materials for the construction stage; information regarding the operation and management stage is even scarcer. This situation leads many newcomers to the field to mistakenly use planning and design guidelines as operation and management manuals, resulting in a series of problems.

I. Problems and Challenges

1. Differences between Planning & Design and Operation & Management:

Planning and Design Stage: Focuses on system layout and technical specifications, providing detailed methods for calculating irrigation water requirements, but these methods are often based on assumptions under the most common conditions.

Operation and Management Stage: Requires consideration of various factors in actual operation, such as soil moisture changes, meteorological conditions, crop growth status, etc. Directly applying methods from the planning and design stage may lead to water waste or poor irrigation effectiveness.

2. Lack of Specialized Operation and Management Manuals:

Beginners find it difficult to locate effective guidance materials specifically for irrigation system operation and management, which presents them with numerous challenges when managing and maintaining irrigation facilities (such as sprinkler irrigation, drip irrigation, etc.).

To improve irrigation and fertilization efficiency and ensure healthy crop growth, it is necessary to develop specialized digital models to evaluate the effects of irrigation and fertilization and formulate reasonable irrigation plans accordingly.

First, to scientifically calculate irrigation water requirements, it is necessary to comprehensively consider multiple factors and follow different scientific decision-making steps and methods at different stages.

II. Planning and Design Stage

It is necessary to select an appropriate irrigation method. Common irrigation methods such as drip irrigation, sprinkler irrigation, and flood irrigation have varying water use efficiencies. Drip irrigation delivers water directly to the crop root zone, reducing evaporation and loss, making it relatively more water-efficient; sprinkler irrigation can uniformly moisten the crop area; flood irrigation often results in significant water waste. For calculation, the crop's potential evapotranspiration can be calculated based on the Penman-Monteith equation combined with the crop coefficient. Then, based on the soil water balance equation and considering factors like rainfall and initial soil moisture content, the actual irrigation water requirement can be calculated. More details are thoroughly introduced in micro-irrigation technical specifications; generally, design work can be completed by following the steps and methods provided in the specifications.

Standard Method for Calculating Irrigation Water Requirement

Determining the Water Requirement of the Irrigation Target

The calculation of irrigation water requirement is designed based on factors such as the least favorable irrigation method at the project site, the maximum water consumption during the crop growth period, and the least favorable meteorological conditions (prolonged absence of rainfall, i.e., assuming precipitation = 0). First, it is necessary to determine the crop's growing season and the maximum irrigation demand during its growth period. The least favorable meteorological condition is the crop's water requirement during summer. In the case of multiple crop rotations, the water consumption of the most water-demanding crop planted in summer is selected. Finally, based on the irrigation system's design parameters, such as irrigation method and irrigation efficiency, the irrigation water requirement is calculated.

In the planning and design stage, selecting which crop growth period serves as the design basis, the correct choice is the water consumption during the growth period when the crop's water demand is highest, which is the design water requirement for the crop. The micro-irrigation engineering design specification, GB/T 50485-2020 "Technical Standard for Micro-irrigation Engineering," directly provides tables for crop design water consumption, such as the following table regarding crop design water consumption intensity (mm/d).

In the design stage, there is no need to consider factors such as vegetables having higher water demands during peak growth periods than during maturity, etc., because you only need to perform calculations according to the specification's guidance. This is because the data in the specification are values derived by domestic authoritative departments after organizing experts for extensive verification.

Considering Soil Type and Texture

Soil water retention capacity directly affects irrigation water requirement. Sandy soils have poor water retention, requiring more frequent irrigation but with relatively smaller amounts per irrigation; clay soils have good water retention, allowing for lower irrigation frequency, but each irrigation amount may be larger. However, in the design stage, you do not need to overthink this. You only need to base it on the soil water retention capacity index determined by soil texture, commonly referred to as field capacity, and control the irrigation amount to be between 60~90% of the field capacity.

Calculating Effective Rainfall

Consider the impact of natural rainfall on irrigation water requirement. When calculating irrigation water requirement, effective rainfall should be subtracted. Effective rainfall refers to the amount of rainfall that can be absorbed by the soil and meet the needs of crops or vegetation.

By consulting local meteorological data, one can understand the distribution of rainfall in different time periods and, combined with soil infiltration capacity and crop water requirement characteristics, calculate the effective rainfall.

In the design stage, there is no need to consider rainfall impact. The reason is that at this point, designing the irrigation system is to ensure it meets the water conveyance capacity demand. The absence of rainfall is the least favorable condition, so designing the system under the assumption of multiple rainless days is certainly safer than a system that overly considers precipitation.

Determining Irrigation Efficiency

Irrigation efficiency refers to the ratio of water actually irrigated to the crop root zone to the water output from the irrigation system. Different irrigation methods and equipment have different irrigation efficiencies.

For example, drip irrigation systems typically have high irrigation efficiency, reaching over 90%; whereas sprinkler irrigation systems have relatively lower irrigation efficiency, generally around 70% ~ 80%.

III. Operation and Management Stage

In the irrigation system operation and management stage, it is necessary to build a crop irrigation water requirement calculation model. A deep understanding of crop characteristics is crucial. Different crops have significantly different water requirements at different growth stages. Therefore, it is necessary to identify the types of crops planted and their current growth stage to accurately assess their basic water needs. Secondly, conduct a detailed analysis of soil properties, including soil texture, porosity, water holding capacity, etc.

These characteristics directly affect water storage and infiltration in the soil, thereby significantly impacting irrigation water requirement. Furthermore, closely monitor local climatic conditions. Climatic factors such as rainfall, temperature, humidity, wind speed, and sunshine duration all affect crop transpiration and soil water evaporation, thereby altering irrigation demand. To scientifically calculate irrigation water requirement, constructing a decision-making digital model suitable for the operation and management stage is necessary. The following are the steps for building such a model:

1. Data Preparation:

Soil Characteristics: Collect information on soil texture, permeability, water holding capacity, etc.

Climatic Conditions: Obtain local meteorological data such as temperature, precipitation, evaporation, wind speed, etc.

Crop Types: Identify the types of crops planted, planting area, growth cycle, water requirement patterns, etc.

2. Parameter Setting:

Start Time of Growth Cycle: Determine the specific time of crop sowing or transplanting.

Division of Different Plant Development Stages: Based on the crop's growth cycle, divide it into different development stages, such as germination, vegetative growth, flowering, fruiting, etc.

Initial Growth Rate: Set the initial growth rate and other relevant parameters according to the growth characteristics of different crops.

3. Model Validation and Optimization:

Comparison of Simulation Results with Field Measurement Data: Test the model's accuracy by comparing the simulated irrigation demand from the model with actual field-measured data.

Feedback and Improvement: Continuously adjust model parameters based on feedback until the model accurately reflects the actual situation.

4. Application Practice:

Assisting Irrigation Decisions: Once the model is adequately debugged, it can be applied in the real world to assist in making irrigation decisions.

Continuous Monitoring and Adjustment: During practical application, it is necessary to continuously monitor the performance of the irrigation system and make timely adjustments based on actual conditions.

In the design stage, irrigation water requirement can be calculated by following the specifications, such as the guidance in GB/T 50485-2020 "Technical Standard for Micro-irrigation Engineering," to complete the design work. In the operation and management stage, scientifically calculating irrigation water requirement is a complex but critical task. It requires comprehensive consideration of multiple factors including crops, soil, climate, irrigation methods, system efficiency, and topography to achieve efficient water resource utilization and sustainable agricultural development. Applying crop digital twin models can effectively enhance the operation and management level of irrigation systems, reduce water waste, improve irrigation efficiency, and promote the development of new quality productive forces in agriculture.