Risk/opportunity:(from the Climate Change Risk Assessment for Scotland 2012):
• AG4 Drier soils (due to warmer and drier summer conditions) • AG25/AG51/AG52 Agricultural land classification and crop suitability
SCCAP theme: Natural environment Buildings and infrastructure
N3: Sustain and enhance the benefits, goods and services that the natural environment provides
Reference period 1981-2000.
The area of prime land classified with moderate/severe drought risk is ca. 2.5% (using winter wheat as an indicator crop)
 Prime land should be capable of growing one of the LCA indicator crops without moderate or severe drought risk. Cereals (as indicated by winter wheat) have lesser water requirements than potatoes therefore are less vulnerable to drought risk. Land not suitable for potatoes may be suitable for cereals and is therefore still classified as prime land, so wheat provides the indicator associated with prime land.
- Drought risk constrains land use options and can increase demand for irrigation water to maintain agricultural productivity.
- In average years the drought risk to arable crops is small, and largely compensated by the availability of water for irrigation.
- Some parts of East Scotland have shown an increase in drought risk, with further increases projected for the future across larger areas of prime agricultural land.
- Land use options may be limited unless irrigation supply is increased; however this could increase water-stress in some catchments in North-East and South-East Scotland.
- As an alternative to irrigation, other forms of adaptation could be developed (e.g. shifts in cropping systems and changes in varieties).
Drought risk is a component of Land Capability (Brown et al., 2011) that identifies constraints on land use options through limitations on water availability. This has implications for the amount of additional irrigation water that may be required to reduce moisture deficits and maintain agricultural productivity. In Land Capability for Agriculture, indicator crops are used to reference drought risk. The main indicator crops are potatoes, cereals (winter wheat/ spring barley), and grass. Of these potatoes is the only crop that is normally irrigated together with other horticulture crops because of their higher economic value. Cereals (as indicated by winter wheat) have lesser water requirements than potatoes therefore are less vulnerable to drought risk. Therefore, land not suitable for potatoes may be suitable for cereals and is hence still classified as prime land, so wheat provides the indicator associated with prime land.
Agro-climatological metrics for drought risk are empirically defined based upon the interaction between climate and local soil properties through the soil moisture balance, notably in terms of available water capacity and its ability to meet the demands of different crops. For the indicative figures, long-term averages are used (20 years is the norm for land capability) but it should be noted that there is significant year-to-year variability and the average figures do not include the frequency of extreme drought events. Hence, this indicator is for extent of drought risk rather than for frequency or severity.
A small area of Scotland that is otherwise well-suited for arable cropping is currently exposed to drought risk due to the limited available water capacity of the soil at these locations (Figure 1a). Therefore, in average years the risk to national agricultural productivity is relatively small (and largely compensated by the availability of irrigation water). However, there is evidence that the drought risk can become more pronounced in extreme years (Brown et al., 2011).
Analysis of past change back to 1961-1980 shows no significant change in average conditions, although there is some suggestion of a small increase in risk for parts of East Scotland due to a shift in temporal averages towards higher soil moisture deficits.
Although future climate scenarios vary in terms of the degree of change that may be expected, the general inference across a range of scenarios is that there is likely to be an increase in drought risk in East Scotland. Analysis by Brown et al. (2011) (based upon two representation UKCP09 SCP scenarios for the 2050s) suggested that 40-50% of prime land may be defined as of moderate or severe risk compared to 4% for baseline (1981-2000) conditions (with winter wheat as the indicator crop). The ‘at risk’ areas are predominantly in East Scotland. For potatoes this magnitude of change would suggest an additional ca. 50 mm of irrigation water would be required during the summer.
Figure 1 Drought risk for wheat a) 1981-2000; b) 2050s (HadRM3 SCP q3 scenario)
Using the same 2050s climate projections, analysis for Scotland’s Centre of Expertise for Waters (CREW) has identified ‘hotspot’ catchments based upon a continuation of current land use and management regimes, with irrigation concentrated on potatoes and horticulture (Brown et al., 2012). These hotspot catchments are particularly evident in Tayside and Fife (Figure 1b). If irrigation was extended to reduce drought risk to cereals, then water-stressed catchments would also become much more common throughout North-East and South-East Scotland. As an alternative to irrigation, other forms of adaptation could be developed (e.g. shifts in cropping systems and changes in varieties).
These results show the key influence of summer precipitation (in combination with evapotranspiration) on drought risk. At present, summer precipitation is variable from year to year and long-term averages show a complex pattern with a slight trend to increased soil moisture deficits in parts of East Scotland which can particularly affect coarse-grained drought-prone soils. This intrinsic (‘natural’) variability acts to confound a clear interpretation of long-term trends.
In the future, the likely trend towards drier summers (on average) shown by most climate projections means that the drought risk is expected to considerably increase and to involve large areas of prime agricultural land in East Scotland. This may limit some land use options unless irrigation supply is increased or other forms of adaptation occur.
In addition to decadal variability, inter-annual variability is also a major factor for land use (or management) decisions associated with drought risk. This inter-annual variability may modify the average conditions (see Brown & Castellazzi, 2014).
The metrics should be considered in combination with other aspects of land capability, notably the overall assessment and wetness risk (see indicators ‘NA2 Area of Prime Agricultural Land (Land Capability)’ and ‘NA13 Abstraction of water for irrigation’).
LCA identifies potential land use and for issues related to water availability, irrigation supply also needs to be considered to evaluate the full land use implications.
Bibby, J.S., Douglas, H.A., Thomasson, A.J., Robertson, J.S. (1982) Land capability classification for agriculture. Macaulay Land Use Research Institute, Aberdeen
Brown, I., Poggio, L., Gimona, A., Castellazzi, M. (2011) Climate change, drought risk and land capability for agriculture: implications for land use in Scotland. Regional Environmental Change, 11, 503-518
Brown, I., Dunn, S., Matthews, K., Sample, J., Poggio, L., Miller, D. (2012) Mapping of water supply-demand deficits with climate change and land use change. Centre of Expertise for Water http://www.crew.ac.uk.
Brown, I. & Castellazzi, M. (2014) Changes in climate variability with reference to land quality and agriculture in Scotland. International Journal of Biometeorology. DOI: 10.1007/s00484-014-0882-9
Land Capability for Agriculture:
Development of this indicator and primary author of this document: Iain Brown, James Hutton Institute.
This research has been funded by Scottish Government through their strategic research programmes: Environment: Land Use and Rural Stewardship (2006-11) and Environmental Change (2011-16).