liminfoCROPWAT Reference
Free reference guide: CROPWAT Reference
About CROPWAT Reference
The CROPWAT Reference provides a comprehensive, searchable guide to the FAO CROPWAT 8.0 irrigation planning software, covering reference evapotranspiration (ETo) calculation using the Penman-Monteith method, crop coefficients (Kc) for all growth stages, and crop water requirement (CWR) estimation for efficient irrigation management.
This reference covers the complete CROPWAT workflow including CLIMWAT climate data integration, effective rainfall computation, soil moisture characteristics, irrigation scheduling options (fixed interval, depletion-based, and deficit irrigation), daily water balance simulation, and yield response analysis using the Ky factor.
Designed for agricultural engineers, agronomists, irrigation consultants, and water resource planners, this tool helps users quickly look up CROPWAT parameters, formulas, and procedures. All content is accessible from any device with no software installation required.
Key Features
- Complete ETo reference with FAO Penman-Monteith equation parameters and input requirements
- Crop coefficient (Kc) values for rice, maize, tomato, and other major crops by growth stage
- Three irrigation scheduling methods: fixed interval, RAW depletion-based, and critical depletion
- Daily soil water balance simulation with TAW, RAW, and root zone depletion tracking
- CLIMWAT 2.0 climate database integration guide with 5,000+ global weather stations
- Yield response factor (Ky) and deficit irrigation strategy analysis for water-saving scenarios
- Effective rainfall calculation methods including USDA SCS and FAO/AGLW formulas
- Scheme-level water supply planning with peak demand and irrigation efficiency calculations
Frequently Asked Questions
What is ETo and how does CROPWAT calculate it?
ETo (reference evapotranspiration) is the evapotranspiration rate from a reference grass surface calculated using the FAO Penman-Monteith equation. CROPWAT requires monthly climate inputs including temperature, humidity, wind speed, and sunshine hours to compute ETo in mm/day. The CLIMWAT 2.0 database can automatically provide these climate parameters for over 5,000 stations worldwide.
How do I determine crop coefficients (Kc) for different growth stages?
Kc values vary by crop and growth stage. CROPWAT uses four stages: initial (Kc_ini), development, mid-season (Kc_mid), and late season (Kc_end). For example, rice paddy Kc values are 1.05/1.20/0.90, while maize is 0.30/1.20/0.35. The actual crop evapotranspiration (ETc) is calculated as ETo multiplied by Kc for each period.
What is the difference between CWR and IWR?
CWR (Crop Water Requirement) equals ETc minus effective rainfall (Peff), representing the net water deficit the crop needs. IWR (Irrigation Water Requirement) divides CWR by irrigation application efficiency (Ea), accounting for water losses. For example, with CWR of 450mm and sprinkler efficiency of 0.75, IWR would be 600mm.
How does CROPWAT calculate effective rainfall?
CROPWAT offers multiple effective rainfall methods including the USDA SCS method, FAO/AGLW method, and a fixed percentage approach (70-80%). The FAO formula uses Peff = 0.6P - 10 for P < 70mm and Peff = 0.8P - 25 for P >= 70mm. Effective rainfall represents the portion of total rainfall actually available to crops after runoff and deep percolation losses.
What soil data does CROPWAT need for irrigation scheduling?
CROPWAT requires soil moisture characteristics including field capacity (FC), wilting point (WP), and the resulting Total Available Water (TAW). For example, loam soil has FC=0.27, WP=0.12, giving TAW=150mm/m. These values determine when irrigation is needed based on root zone depletion relative to Readily Available Water (RAW = p x TAW).
How does deficit irrigation analysis work in CROPWAT?
CROPWAT uses the yield response factor (Ky) to estimate yield reduction from water stress. The formula is (1 - Ya/Ym) = Ky x (1 - ETa/ETm). For water-sensitive crops like maize (Ky=1.25), supplying 80% of water needs results in approximately 18.75% yield loss, helping planners evaluate water-saving versus yield trade-offs.
Can CROPWAT handle multiple cropping systems?
Yes, CROPWAT supports multiple cropping and crop rotation scenarios. You can define sequential crops (e.g., rice from April-August and wheat from October-March) and the software aggregates total annual irrigation requirements across all cropping periods, allowing comparison with available water supply capacity.
What are the three irrigation scheduling options in CROPWAT?
Option 1 uses fixed intervals (e.g., every 7 days with set application depth). Option 2 triggers irrigation when soil moisture depletion reaches the RAW threshold, applying water to refill the root zone. Option 3 uses a fixed application amount when critical depletion is reached, enabling deficit irrigation simulation with yield reduction estimates.