CFT Water Assessment Description

Transcription

1 CFT Water Assessment Description Cool Farm Alliance 2017 For more information, see Cool Farm Alliance Community Interest Company The Stable Yard, Vicarage Road, Stony Stratford, MK11 1BN England. Website: 1

2 Contents Introduction... 2 Overall Basis... 2 In more detail... 3 In even more detail... 3 Results... 5 Data Required... 5 Future... 5 Introduction The Cool Farm Tool Water Metrics assessment (CFTw) enables farmers and their supply chains to assess their water demand, water consumption and irrigation efficiency with standard crop data, using localised meteorological information. From this the CFTw produces a WFN compliant blue and green water footprint and a crop/soil water balance. At this stage, it does not produce a grey water footprint. The CFTw follows the same modus operandi as the GHG tool, and is embedded within that, requiring a few extra inputs. The purpose of this document is to explain what is taken into account and how the algorithms are built up. It is intended as a lay document for aware but not scientifically detailed readers. For further details and references please see CFT Water Assessment Technical Document. Overall Basis CFTw is based on the FAO56 standard, first published in This is based on a threestep process Estimating a baseline standard evapotranspiration for a location and atmospheric climatic conditions. A short well-watered grass crop is used as reference/start point. Scaling this for the crop in question under ideal crop growing conditions (i.e. without considering stress and local weather conditions). Applying adjustments to take into account soil conditions, water stress and correcting crop factors for local weather/climate. Global datasets are used for climate (ERA Interim), soil (Harmonised World Soil Database and FAO56) and crops (FAO56). The FAO56 model has been enhanced to further improve soil water dynamics by inclusion of run-off, interception, soil organic matter etc. The scope of the tool is to work at crop/field level. Assessing whole farms or river basins requires the assessment of each individual field. 2

3 Other models: The CROPWAT model is also based on FAO56 and takes a comparable approach, but whereas in CFTw, the tool provides soil data (via Harmonized World Soil Database) and climate data (via ERA INTERIM) so that the user only needs to provide the crop, in CROPWAT, the user must provide soil and climate data as well as crop data. Another model provided by the FAO is AQUACROP which is more advanced than CFTw in some areas, however it does require significantly more data that is not widely available at global level and requires a formal licensing agreement when used within CFTw. AQUACROP may be of greater use in specific cases where more data is available and more detail is required by agronomists and/or policy makers. In more detail Using a standard of a short well-watered grass crop as a reference, the model first applies atmospheric forcing parameters that drive water from the plant and soil into the atmosphere. These are daily radiation, temperature, humidity and wind speed values. This produces a reference evapotranspiration (ET) value for the location. A crop factor is then applied to the calculated reference ET value. This factor is specific to the crop in question, and is based on FAO56 data which are empirically based on different crops. It is assumed that there is no water or nutrient stress on the plant. This crop factor is then further adjusted by considering local climate data (weather) for humidity and precipitation. Crop height is also factored in as are water stress and management practices to assign a realistic ET value. In even more detail 1. CFTw uses the concept of crop and a field and run in daily time steps 2. The reference ET value is calculated using a modified Penman-Monteith equation. 3. The technical document further expands this to show how each of the required parameters is also determined, and these parameters are: a. Radiation at the crop surface b. Soil heat flux density c. Mean daily air temperature d. Wind speed at 2m e. Saturation vapour pressure f. Actual vapour pressure g. Vapour pressure deficit h. Slope vapour pressure curve i. Psychometric constant 4. The adjustment to the reference ET value are given below and algebraic details for these are in the technical document. 5. To determine the crop factor adjustment CFTw uses a single crop coefficient curve representing the development of the crop based on four phases: initial, development, mid-season, late-season. Timings for these will vary by crop type. Values for these are provided by FAO During the initial phase ground cover is small and so evaporation from bare soil is 3

4 key. Depending on wetting interval of the soil the evaporation is moisture or energy limited. A frequent wetting is energy limited, because moisture is abundant, thus the crop factor is high. If irrigation is taking place, then the soil is typically always wet. 7. Crop factors for the mid and late period of the growing season are mostly dependent on crop. 8. The correction of the crop factor for the local climate for the later phases is happening in the third step, where the default crop factors are adjusted for local humidity, wind speed and crop height. 9. Water stress in the root zone will also impact ET values and are taken into account with the CFTw. The level of stress is derived by the soil water balance in the root zone considering precipitation, irrigation, deep percolation, runoff, interception and ET. Irrigation depth, itself is a function of amount, runoff and leaf canopy cover. The loss through interception (from leaf canopy) is determined according the Waganingen SWAP model. This also takes account of the crop development using a crop specific average Leaf Area Index from the literature. Runoff is determined according to the LPJmL approach described in Jägermeyr et al CFTwtake into account different farmer management practices. It is intended that thus section will grow over time as more practices are a) recognised, b) catered for within the tool. The initial version of CFTw considers different irrigation methods, as they affect the soil water balance and are an important tool for farm water management. At present four are allowed for: a. Pivot b. Rain gun c. Flooding d. Drip 11. Interception losses are calculated for top down irrigation: pivot and rain gun. Use of irrigation affects soil water stress and so overall ET values. The CFTw applies equations to capture and allow for this. 12. Soil organic matter (SOM) increases the amount of water that can be held within the soil and makes crops more resilient against droughts. This relationship between SOM, soil texture and available water is held algebraically within CFTw (Saxton and Rawls 2006 ) but assumes the outcome is constant for the rooting depth (may need to change for perennials). NB. In both all the CFTw approximates dates to ease farmer record keeping/sentry by asking for start, middle and end of moth dates, and these are translated to the 5th, 15th and 25th of each month respectively. 13. In summary actual ET is the result of the following: ET = reference ET for the region/year x crop factor & development stage x adjust crop factors for local weather x water stress x management intervention x soil texture/organic matter 4

5 Results From all the above calculations CFTw derives the blue and green water footprints, according to Hoekstra and WFN (reference). The net irrigation requirement is calculated by looking at the crop potential ET without considering water stress and precipitation, with a positive value indicating that irrigation is required. This is based across the whole growing season and so may not fully account for periods of very heavy precipitation causing abnormally increased runoff or percolation during that period. The soil water balance table shows the determined factors as soil water balance of Adjusted crop ET (plants water consumption) Precipitation (rainfall) Deep percolation (what soaks in and not use by the plant) Runoff (surface water escape Capillary rise (not considered in this phase, but th water which rises from the water table by capillary action) Irrigation (the water added by the farmer) Data Required Five data sources support the CFTw these are: ERA Interim Dataset Harmonised World Soil database FAO56 standard Scientific literature & others Input Data, primarily sowing date, harvesting date, soil data and irrigation management information Future Future version of CFTw are expect to provide: More management options including fertigation implications and its links to CFT GHG assessment calculations More crop types, including perennials where suitable data sets sexist Application of catchment water availability data ( scarcity ) More details in the soil water balance parameters Aggregation of field/crop to at least whole farm level. 5