Vegetation and Ground Water – a Two-Way Relationship

by Cate Moore on January 13, 2014

It’s another perfect day in Paradise; a sunny day with mild temperatures that allow me to work outside in shorts and a T-shirt.  The only problem with this is that it’s January, and we’ve been experiencing weather akin to mid October when we should be collecting our allotment of rain for the year.

As I write this, our weather station has only recorded 2.21 inches of rain for the rain year, and we have not seen truly significant rain since December 2012.  It isn’t a comfortable feeling, living in a bone-dry forest and waiting to see if our next major event will be the welcome return of the rain or the feared sweep of a fire.

The state of the ground water supply on forest growth is well established.  Studies conducted by UC Extension forestry tested forest tracts to see what limitations had the greatest effect on forest growth.  The elements tested were sunlight, water, and soil fertility.  The most potent limiter to tree growth, hands down, was a lack of water.

The effect of vegetation cover on the ground water supply receives a great deal less attention.  We don’t directly observe what’s going on under ground in our daily routines and it is easy to forget that there is a lot of activity going on under our feet.  Let’s explore together how much a tree drinks in a year, then use this information to explore the effects of vegetation on our ground water supplies.

The process of photosynthesis is described in Is a tree a heavy drinker or does it just pump water?  The article notes that to generate a pound of cellulose (the main constituent of wood), the tree binds 0.55 pounds of water to the product and releases more than 90 pounds of water into the atmosphere through transpiration.  At 8.33 pounds of water per gallon, we calculate that it takes every tree, shrub and flower 10.87 gallons of water to create one pound of wood.

Redwood Empire’s website FAQ section answered the question of “How much do Redwood Trees weigh?” by noting that “A … typical farmed Redwood tree will weigh approximately 50,000 pounds”.  Local forest management growth predictors use a 3% per year figure for estimating how much our central coast forests grow.  This means a typical farmed redwood will put on 1500 pounds of wood in a year, using 135,825 pounds or 16,306 gallons in the process.  Most of this water is drawn during the May through August growing season, when the water table is saturated and a generous amount of sunlight is available to power the photosynthesis chemical reaction.

This is just one tree.  What happens when we look at this water draw on a landscape scale?

Former CCFA board member and scientist/engineer Robert O. Briggs examined this question in the Waddell Creek watershed in his study Competition for limited dry season ground-stored water between forest use and stream flow in the Waddell Valley.  He characterized the Waddell watershed as a system that is entirely fed by rainfall in the rainy season and noted that in the Waddell Creek, the agricultural draw is not a significant part of the observed situation, since the agricultural diversions take place downstream of the flow rate monitoring stations.  This leaves reforestation as the driving force of the differences in flow rates observed over time.

In this study, Theodore Hoover remarked that in 1913, Waddell Creek flowed 1800 gallons/minute in September, and the lowest flow he had ever noted was 1200 gallons/minute in a drought.  During the drought year 1976-1977, Robert Briggs measured the Waddell Creek flow rate at 76.5 gallons/minute.  He did a more systematic set of stream flow measurements between the years 1988-1998.  Mid-range data for the years 1933-1942 came from Shapovalov and Taft’s anadromous fish study, The Life Histories of the Steelhead Rainbow Trout (Salmon Gairdneri gairdneri) and Silver Salmon (Onchrhynchus kisutch), where part of the data gathered included continuous flow rates for the creek.

Robert Briggs then compared the flow data from the 1933-1942 period with the flow data from the 1988-1998 period and determined that the effect of the revegetation of Waddell Creek watershed for the 55 year span was equivalent to losing 18 inches of rainfall in a year.  He established a robust relationship between the degree of forestation on a watershed and the watershed’s available ground water.

Water supply calculations and water policy decisions seldom reference vegetation loads as a significant factor in the state’s overall water management plan.  This is a crucial omission.  Millions of gallons of water that might otherwise be utilized to keep streams flowing, water crops and provide domestic water are instead being cast uselessly into the air by the state’s overstocked vegetation.  Is a Tree a Heavy Drinker or Does It Just Pump Water? discusses the tradeoffs between water quality and water yield experienced by water management districts that plant trees in the watersheds that feed their reservoirs.  Trees stabilize soil and keep water cool, but they also exact a price for their services in reduced water yield.

Due to the extraordinary water costs of photosynthesis, we must also ask the question of which is more important to the welfare of California, carbon sequestration or adequate water supplies?  Dry wood is about half carbon, and, since nearly 11 gallons of water is needed to generate each pound of wood, we can calculate that it takes over 21 gallons of water for each pound of sequestered carbon.  Once again, a portion of this water is locked into the wood, but the bulk of it is released into the atmosphere by transpiration and is no longer available for other purposes.

Like all real-world situations, choosing the best path will not be easy. The decisions that set the path will require tradeoffs and value decisions at every step of the way, and while we are making these choices, it behooves us to remember how our forests affect our ground water supplies.

BRIGGS, ROBERT O., 1999.  Competition for limited dry season ground-stored water between forest use and stream flow in the Waddell Valley. Unpublished manuscript

DECOSTER, LESTER A. AND JOHN HERRINGTON, 1988.  Is a tree a heavy drinker or does it just pump water?   American Tree Farmer.  May-June pp. 17


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