NA2 Area of Prime Agricultural Land (Land Capability)

The total amount of prime land has stabilised in the most recent reference period (1991-2010), although this masks small increases in east Scotland (notably north east Scotland) and small losses in south west Scotland (Figure 1).

Figure 1 LCA summary for prime land and improved land (a) 1961-1980 (b) 1971-1990 (c) 1981-2000 (d) 1991-2010 [1:250,000 scale] (Brown & Castellazzi, 2014)

Analysis of past change has been made using reference periods back to 1961-1980.  Over this time period, the percentage of land in Scotland defined as prime land has increased by ca. 4% (this is ca. 18% expansion in prime land area excluding wetness risk)  but most of this expansion occurred during the period 1971-1990 and there have only been smaller changes since then (although with distinctive geographic variations).

Analysis using both UKCIP02 (Brown et al., 2008) and UKCP09 (Brown et al., 2011) indicate a significant expansion of prime land, identified as being in the range of 20-40% (Figure 2). This is mainly a consequence of the shift towards warmer drier summers for the lowland agricultural areas of Scotland (in average years) evident in most climate change projections or scenarios.

Figure 2 Changes in LCA (a) official 1:250,000 published version (1982)  (b) 2050s (UKCP09 SCP ensemble mean) (Brown et al., 2011)

Land capability is influenced both by temperature increases and soil moisture deficits (i.e. drier conditions) which in Scotland are both associated in general with improved agricultural productivity and flexibility of land use. Temperature increases act to extend the growing season whilst drier soils reduce wetness risks (NB. if soils become too dry then they are classified to be higher drought risk but this is assessed separately as it is crop-specific).

The increase in temperatures  over recent decades has improved land capability and the area of prime land but this has been complicated by changes in soil moisture deficits (linked to summer precipitation) which have been more variable (spatially and temporally).

In the future, the likely trend towards warmer drier summers (on average) shown by most climate projections means that a large expansion of prime agricultural land is anticipated. However this may be modified by an increased drought risk (see ‘NA29 Drought risk to agricultural land’ indicator).

In addition to decadal variability, inter-annual variability is also a major influence on land capability and there is some evidence that some areas have experienced increased variability in yearly conditions, notably in SW Scotland (see Brown et al., 2014). This is currently not included in the indicator

The metrics should be considered in combination with other aspects of land capability, notably drought risk and wetness risk, which may cause a downgrading of prime land status, also depending on adaptation assumptions made for irrigation and drainage respectively.

Other indicators, such as the area of land that may be potentially improved for agriculture, are also available. They show similar overall trends but with different spatial variations that can be important for land use options in some regions.

The following issues should also be noted:

• LCA is a classification or model. As such, the mapped output does not exist on the ground as say a wheat field, soil type or a house does.
• The suite of maps presented here is the result of modelling in a GIS environment; the area figure of extent has been derived from maps based on assessment in the field albeit using the same set of guidelines. These are different methodologies; GIS modelling is more systematic and objective but does not capture the nuances that field work can capture.
• The projections into the future do not capture important soil-climate interactions. The predicted future extent of prime land in the 2050s presented here is likely to represent the maximum possible extent.
• Like all maps displaying features of the environment, care must be taken in their interpretation and consideration given to the resolution of specific maps. Obvious features on the ground often simply cannot be represented on maps of any scale. LCA maps are indicative and rely heavily on the underpinning soil information and its interpretation.
• LCA has a number of underpinning assumptions covering aspects of land management, investment, farm structure and social aspects such as farmer’s age and distance to market.
• Parcels of land identified as single LCA class are highly likely to contain land of lower or higher quality; indeed this aspect is built directly into the classification guidelines. The higher the quality of land, the less likely it is to include land of poorer quality
• An LCA assessment is based on climate averages over decades; it does not capture year to year variation which can be quite extreme. This variation becomes greater in the lower land classes. As such, prime land is more ‘stable’ year by year but the climate in Class 2 and 3.1 land can be more representative of lower classes in a number of years.

Notwithstanding these caveats, LCA does provide a robust and straightforward way of assessing the inherent quality of land for agriculture and how agriculture might adapt in a changing climate. The data presented here is best applied at the national and regional levels and has been in a number of previous applications. The system was published in 1982 and the mapped output shortly afterwards; both are viewed with some degree of respect by a whole suite of relevant stakeholders who are aware of the strengths of the system, but also some of its constraints. Used appropriately, LCA is a good indicator and consideration should be given to extending its use beyond that of prime land, for example Classes 3.2-5.1 are the backbone of the Scottish livestock industry and there are climate change implications for those classes too.