The British Met Office has recently provided spatially accurate (2.2 km resolution) climate projections for England in its latest UKCP. They have followed a new “Convection-Permitting” Model, which allows reducing the spatial and temporal scales of regional climate projections. The daily meteorological data, such as temperature and precipitation, at 5-km grid cells, could help to estimate future irrigation needs by 2030, in farms growing rainfed cereals currently.
The UKCP local projections have been downscaled from the Met Office Hadley Centre’s global climate model. The projections consider the RCP 8.5 emission scenario from the IPCC AR5.
Local (2.2 km) better captures the spatial structure of rainfall, including local showers.
Cereals, such as wheat and barley, mean more than 50% of the total British cropping land. Wheat and Barley are rainfed crops, usually. Wheat is the most frequent cereal. It is particularly important in regions such as the East Midlands and East of England. Global warming will bring higher temperatures at the world scale. Therefore, we could wonder if rainfed wheat and barley could be affordable in the future.
Estimating wheat future irrigation needs
Irrigation needs and/or nitrates leaching depend on soil water movement up/down. This movement relies on soil water content and soil hydraulic properties. Meteorological variables, such as temperatures and precipitation, are crucial since evapotranspiration and soil percolation depend on them. Particularly, precipitation variability is the main variable to consider. Dry spells, i.e. several consecutive days without rain, can affect crop yields in rainfed agriculture. Likewise, heavy rains bring percolation and nitrate leaching, especially if high concentration of soluble nitrates are present in the soil solution.
We selected the 2030 wheat crop season in one location of East England, as well as a soil type with reliable fertility levels (data from LandIS). We followed a simple approach for estimating irrigation needs. Furthermore, we considered the simple Hargreaves method for evapotranspiration calculations. This method will require only maximum and minimum temperatures, since radiation depends on location. We downloaded daily temperatures and precipitations from UKCP local projections for the 5 km grid cell at 547500.0, 262500.0 for the 2021-2040 time range, although we only consider from autumn 2030 to summer 2031, which is the typical wheat crop season.
Figure 1 shows the daily precipitation for each of the 12 members of the local projection. There is a clear variability among the members, in several cases extreme events.
The table shows the estimated wheat mean (ETc mean), maximum (ETc max) and minimum (ETc min) evapotranspirations, as well as its deviation (ETc des). We show the average (ADS) and maximum (MDS) dry spells (i.e. days with precipitation lower than 0.1 mm). The table shows also the average (ADBFC) and maximum (MDBFC) days when soil water content was below Field Capacity.
| Member | ETcmean | ETcmax | ETcmin | ETcdes | ADS | MDS | ADBFC | MDBFC |
|---|---|---|---|---|---|---|---|---|
| p00000 | 1.84 | 8.05 | 0.24 | 1.66 | 3.7 | 10.0 | 7.8 | 29.0 |
| p01113 | 1.83 | 5.90 | 0.17 | 1.57 | 4.1 | 17.0 | 11.4 | 29.0 |
| p01554 | 1.76 | 6.60 | 0.24 | 1.76 | 3.4 | 8.0 | 11.9 | 34.0 |
| p01649 | 1.70 | 5.80 | 0.17 | 1.70 | 3.3 | 12.0 | 8.7 | 23.0 |
| p01843 | 1.70 | 6.40 | 0.26 | 1.40 | 2.9 | 7.0 | 7.6 | 51.0 |
| p01935 | 2.05 | 9.39 | 0.27 | 1.88 | 4.3 | 15.0 | 8.9 | 67.0 |
| p02123 | 1.86 | 7.62 | 0.12 | 1.69 | 3.9 | 9.0 | 10.5 | 35.0 |
| p02242 | 2.06 | 9.34 | 0.18 | 1.99 | 4.7 | 16.0 | 11.1 | 33.0 |
| p02305 | 1.86 | 7.60 | 0.24 | 1.62 | 3.0 | 6.0 | 9.5 | 32.0 |
| p02335 | 1.71 | 7.07 | 0.20 | 1.49 | 4.0 | 8.0 | 8.4 | 32.0 |
| p02491 | 1.88 | 6.58 | 0.28 | 1.49 | 3.0 | 10.0 | 9.9 | 45.0 |
| p02868 | 2.01 | 7.48 | 0.29 | 1.77 | 3.2 | 12.0 | 9.5 | 23.0 |
In many cases, the combination of relative high temperatures and lower precipitations brought a relative higher number of days when soil water content was below field capacity. Theoretically, soil water contents below field capacity means the soil pores are partially empty.
This indicates a potential irrigation need as well as eventual yield loses in rainfed conditions. One of the members, p01554, shows both extremes: very high occasional daily precipitation and a relative high dry spells and days below field capacity.
Fig. 2 shows the average soil water contents, as well as maximum and minimum, considering all the 12 members of the local projection. The average soil water contents are below field capacity in few occasions. Lowest soil water contents are found in summer, at the end of the wheat crop season, when this might not be particularly significant. Even the minimum soil water contents, considering all the members, are not very much below field capacity, relatively. Actually, field capacity is not the right trigger of irrigation, but a percent of the difference between field capacity and wilting point.
Therefore, irrigation might not be needed in wheat farms by 2030. Fig. 2 indicates that average soil water contents, considering all the member of the projection, are higher than field capacity most of the time. This means that flooding and nitrate leaching could be the main risks in wheat fields by 2030. The particular situation at each farm depends on soil properties.
Results suggest that water excess rather than soil water deficit should be expected in wheat fields in central England by 2030. The main risk could be nitrate leaching, especially in regions already declared as vulnerable zones.
The analysis conducted here is very simple. A more thorough assessment should be based on crop models and simulations. For instance, simulations with the agrohydrological model SWAP could estimate daily percolations, which seems to be an important component on water balance, according to precipitation rates. This simulated percolation could be used to estimate nitrate leaching and the eventual groundwater pollution.