The Water Cycle – where do all the water and nutrients go
The water cycle is a series of flows and storage components from rain to evaporation. The soil type on a farm determines the path the water takes from when it hits the ground until it evaporates. Researcher Owen Fenton gives information on where the water and nutrients go.
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Most people will know of the water cycle from their secondary school geography textbooks (see below). This important cycle is a series of flows and storage components. Firstly, precipitation falls onto the Earth surface where it is intercepted by vegetation or flows along surface pathways as overland flow (then runoff) or infiltrates into the soil and percolates deeper to groundwater. This groundwater then migrates and interacts with surface water or feeds coastal waters. Water evaporates from water surfaces or through transpiration from plants and eventually condenses falling once again as precipitation. And so the cycle is continuous.
At farm scale soil type is very important in terms of which pathway water takes (surface or subsurface or both) and what nutrients (nitrogen and phosphorus) follow this water. Therefore, the same amount of water falling on two different farms can be divided up very differently between surface and subsurface pathways. Indeed, different parts of a farm can behave differently.
Well drained systems
In an area with free draining soils and subsoils underlain by Karst limestone or shallow low permeable bedrock (causing lateral shallow movement of water), infiltration is high and overland flow is low. Here water is allowed into the soil and there is an absence of land drainage and open ditches. The dominant pathway of water and nutrients dissolved in that water will be through the subsurface and along groundwater pathways. The nutrient of most interest will be nitrate and above all diffuse source control will be important to minimise surplus nitrogen. Knock on changes to water quality in such systems will take years to decades i.e. there is a time lag. Incorporation of time lag principles into future water quality regulations will provide regulators with realistic expectations when implementing policies. In Ireland, the Nitrates Directive is the main programme of measures in place to meet the goals of the European Union Water Framework Directive.
Poorly drained systems
In an area with poorly drained mineral or peaty soils and underlying poorly productive aquifers, infiltration is low and overland flow is high. Here the soil impedes infiltration and there is typically artificial drainage installed with an associated open ditch network diverting water to an outlet. The dominant pathway of water and nutrients dissolved in that water will be though surface overland flow pathways and through shallow artificial drainage systems. The nutrient of most interest will be phosphorus and in drainage systems more than likely nitrate has been converted to ammonium. In terms of mitigation of such nutrients loads the pathway must be broken or intercepted. In terms of time lags such systems take shorter times to react after breaking the pathways of loss and this is typically in the order of weeks to months.
Take home message
Knowledge of the water cycle and soil drainage type at any location on an Irish landscape will help you form a conceptual picture of where water is likely to travel. This will inform you of the journey water takes and the timescales involved along surface or subsurface pathways. At farm level it will help you understand what nutrients travel within these pathways and what measures and where implementation of such measures can mitigate such losses.
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