North-facing slopes: Topography and the start of spring
Topography describes many aspects of the physical landscape we live in. February Map of the Month shows areas where topography has a tangible impact. Soil temperature is key to start of growth in spring. Besides latitude, exposure to sunlight is an important factor determining the start of spring.
View the map here: North-facing slopes: Topography and the start of spring (PDF)
Cartographers: Dr Jesko Zimmermann & Dr Stuart Green
Topography describes many aspects of the physical landscape we live in. The topography is also intrinsically linked to the natural forces that shape the surface of the earth, as well as human life (as shown in an earlier Map of the Month, showing how elevation is linked to stocking density).
With regard to the natural forces, topography is both defined by, and defines how these forces interact with the land. Climate and hydrology, for example, have shaped the surface through weathering and erosion, where rivers cut deep valleys into the ground, and even mountains are slowly ground down (impressively shown in the Burren). Parts of Scandinavia are still rising, after being pushed down by the sheer weight of the glacial ice sheets of the last ice age.
At the same time, the surface influences the weather we experience, as clouds precipitate their excess water as rain as they cross mountain ranges, leaving the land beyond drier. Rivers follow the gradient of the landscape as defined by slope and shape, while at the same time imposing their influence on the land surface.
The island of Ireland was significantly defined and moulded by the last ice age, and is still rich in glacial land forms, including the rolling drumlins of the border counties, glacial lakes such as Coumshingaun Lough (Co. Waterford), glaciated valleys, and even a fjord on the coast of Co Galway. Beyond these, Ireland is also renowned for the many mountain ranges, and steep cliffs. And in many areas, topography has had a direct impact on the timing and practice of agriculture.
The specific measures of topography we are covering in this map are elevation, slope, and slope aspect (i.e. which direction the slope is exposed to).
Elevation and slope define the shape of hills and mountains. They can be high or low and steep or gentle. Elevation in particular has both a direct and indirect impact on the weather. The adiabatic gradient describes the increase or reduction in air temperature due to changes in air pressure, influenced by elevation. The changes in temperature and pressure also change the capacity of air to hold water. As air rises up a slope this capacity decreases, resulting in precipitation.
Slope also has considerable impact on the natural landscape. It determines the catchments of rivers, water retention, and influences nutrient and pollutant flows. It also impacts the ability of soils to form at a location, and determines the accessibility of landscapes to humans.
The aspect, or the cardinal direction of the slope, also has an important influence on local climate. In the northern hemisphere, north-facing slopes receive less direct exposure to the sun, therefore being generally cooler, and more humid, while south-facing slopes are warmer and drier. This has direct implications for agriculture, where south-facing slopes are generally preferable for growing a large variety of crops.
The particular example of the impact of topography on agriculture presented in this month’s map is the start of spring grazing. The driver of growth is soil temperature, and sufficient temperature is required for grass growth to occur. This is mainly seen in the change in cattle turning out date with latitude. Research has shown that the start of the grazing season in Ireland is delayed on average by a day for every 16km travelled northward.
But soil temperature is not only affected by latitude. As highlighted above, local climate is intrinsically linked to topography. Soil on higher, north-facing slopes will warm up slower, effectively slowing the start of spring. The effect has not been quantified on local scales. However, in this map we show areas with a slope greater than 5 degrees (or 8.75 %), facing northward (293 to 67 degrees), and greater than 100 m above sea level (in order to exclude micro relief in the midlands), where topography will likely have an impact on the optimal turn out dates for livestock.