Sea Under the Land

Unlike river discharge or snow cover groundwater is harder to quantify and monitor, precisely because it is under the ground. Yet it is our largest accessible store of freshwater. therefore, we need to understand how agricultural practices are affecting it.

Bumpiness shows variation in gravity.
GRACE satellites are those in the top right hand corner
 Image source: CEOS EO handbook

We can retrieve information about groundwater levels using piezometers, which will provide information concerning one aquifer or part of an aquifer. Alternatively, to obtain a wider picture we can use the satellites of GRACE (NASA's Gravity Recovery and Climate Experiment mission). From GRACE we get an estimate of terrestrial water storage (TWS) which consists of snow, ice, permafrost, surface water, soil water and groundwater. In order to obtain an estimate of groundwater we first calculate the quantities of the other components using remote sensing, models, and ground based measurements THEN subtract these from the TWS. Here is a really clear document to read explaining more about how GRACE satellites work and how groundwater data can be retrieved.

So how can agriculture impact globally upon groundwater? This will be a bit longer than my average post so have a cup of tea and a mince pie handy!

Thirsty Crops:

Roughly 70% of groundwater abstracted is used for irrigation. When abstraction exceeds recharge then groundwater depletion, the permanent loss of groundwater, can occur. 

The risk of groundwater depletion occurring is especially high during periods of drought. Castle et al., (2014) studied the changes in surface water storage and groundwater storage in the Colorado Basin during drought from December 2004 to November 2013 using GRACE data for TWS, Land Surface Models for soil moisture estimates, SNOWDAS data for snow water equivalent, and reservoir storage data from U.S. Bureau of Reclamation. This period was one of the driest that the area has seen. 

Monthly volume anomalies (changes in volume) for groundwater storage (black line) and surface water storage (green line) with errors (shading) for A) the whole basin and Lake Powell and Lake Mead combined  B)  the upper basin and Lake Powell C) the lower basin and Lake Mead. Source: Castle et al., 2014

The results show that the groundwater store of the Colorado basin risks becoming depleted. Groundwater levels have been decreasing as a consequence of successive droughts reducing recharge and driving higher levels of groundwater abstraction due to drought surfacewater allocations not meeting water demands. The Colorado basin the world's most over allocated catchment and with potential increases in drought severity and occurrence due to climate change we are going to see higher levels of groundwater abstraction in the Colorado basin. Joodaki et al., (2014) using the same method highlighted a similar groundwater situation occurring in the Middle East, which indicates this is a situation we are likely to see happening across the globe as climate becomes more variable.

Land Change:  

Source: http://www.sciencemag.org/site/feature/misc/
webfeat/soilmap/soil_austral_links.html

One of the most dramatic situations illustrating the complex relationship between land use and groundwater can be seen in Australia. In parts of Australia groundwater is very saline and the original vegetation, which was deep rooted trees, kept groundwater levels low and in equilibrium. Deforestation of the land and replacement with agricultural crops (that are shallow rooted) led to a huge rise in the level of the water table, bringing all the dissolved salts to the surface, the consequence of which was the salinisation of large swathes of land rendering it unusable. No vegetation can tolerate the salinity; this situation is known as dryland salinity and is still a major problem for Australia.

Another example is the type of crop grown on the land as different crops have different water requirements. Esnault et al., (2014) have undertaken detailed study to determine which groundwater irrigated crops could 'stress' an aquifer system further by looking at the 'groundwater footprint'. The groundwater footprint is the area needed to sustain groundwater use.

The results for the Central Valley and High Plains aquifer systems in the USA are:

The contribution of each crop type to the regions to groundwater footprint.
The colours for the High Plains aquifers (f, g, and h) are different. Source: Esnault et al., 2014

The pie charts shows each crop type's contribution to the region's groundwater footprint to area ratio. For the Central Valley system hay has the largest ratio and for the High Plains system it is corn gain and cotton. Such information can be highly useful in terms of managing groundwater resources by understanding the consequences of growing a particular crop in a certain region. 
Another point worth noting is that the corn grain and hay grown in the regions of study are mainly for the meat industry.... which makes me think more on the issue of whether you can reconcile being an environmentalist and eating meat brought up in the blog Sown on Stony Ground.

With modelling I don't want to present the results without showing a bit of what's going on 'behind the scenes'. 

Esnault et al., (2014) 's study is complex  requiring data on:

•  irrigated and harvested crop areas
• proportion of crop irrigated by groundwater
• efficiency of irrigation system
• management of surface and groundwater
• groundwater abstraction rates

as well as using global and local hydrological models to:

• obtain recharge estimates
• obtain environmental flow requirements 
• obtain net surface water (called blue water) requirements 

The margin for error grows with each parameter added in, however, to certain extents this cannot be avoided as all the information and processes listed above are needed to calculate groundwater footprint. To be aware of the uncertainty in their approach the authors quantified the amount of uncertainty that each parameter contributed:

The actual value and relative contribution to total uncertainty for each parameter for (a) Central Valley and (b) High Plains aquifer systems. Source: Esnault et al., 2014

By doing this the authors clarified where the largest proportion of uncertainty in their estimates of groundwater footprint lay. For this study it is concerning the quantification of irrigation system efficiency and in estimating groundwater recharge using models. 

This leads me onto my next post which will be about groundwater modelling and what we mean by uncertainty.  

Before I go i'll leave you with this interesting conversational tidbit for Christmas dinner:  in the tropics it is not the duration of rainfall but the intensity that is important for groundwater recharge. As such, we might see groundwater playing an important role in human adaption to climate change. 

With that food for thought have a very MERRY CHRISTMAS! 

Source: https://bongous.com/merry-christmas/