Thirsty trees may contribute to declining minimum flows
There are wet years and dry years, and flow in the Mattole river varies correspondingly, but the US Geological Survey gage record shows a declining trend in minimum flows since about 1960, with a faster decline since about 1990. Minimum flows have decreased by roughly ten cubic feet per second (cfs), or about 40%, over the last 25 years. Ten cfs is a lot of water, enough to fill a 2,500 gallon tank in about half a minute, or cover almost 20 acres a foot deep in a day. While there is no downward trend apparent in spring or early summer flows (while variant upon spring rains), flows decrease rapidly through August and the rest of the dry season. Minimum flows, therefore, tend to be lower, for several reasons.
If the fall rains arrive late, there is more time for the flow to drop. August and September rains have been less frequent in recent decades, so the minimum flows also tend to occur somewhat later in recent decades, but analysis of the gage data shows that this effect is minor.
Diversions by marijuana growers receive substantial press. With the “Green Rush,” there are more people and irrigated plants in the basin, and more water is diverted from springs or streams to supply their needs, but the decline in late summer flows at Petrolia seems too big for diversions to be the main cause. Reliable numbers on water use by marijuana cultivation are available, but based on data for Scott Valley in Siskiyou County from UC Davis, one cfs will irrigate about 95 acres of alfalfa in July, or about 160 acres in September—probably equivalent to many greenhouses. Also, irrigation is not new; until the 1970s, ranchers grew irrigated alfalfa on some of the terraces along the river for hay. Diversions do, however, have locally severe effects in tributaries and the upper river, and pumping can dry small streams.
Increased water use by trees, primarily Douglas-fir and grand fir, may be a large contributor to the decline in minimum flows. The forest in the Mattole basin is rapidly recovering from the logging boom of the 1950s and 60s, and the forest has expanded. Mapping by the MRC shows that about 55% of grasslands have been lost between 1950 and 2005. Douglas-fir are also expanding into oak woodlands in interior parts of the Mattole basin, as has been documented in many areas, from the North Fork Eel River to as far north as Vancouver Island. (The MRC is developing an oak woodlands project to quantify and hopefully reverse the process here.) Closer to the coast in the Mattole basin, old, open-grown fir, whose lower branches have been shaded out but have not yet fallen, show that dense young forest has replaced scattered
trees in many areas.
Increases in summer flows after forest clearing have been documented in many kinds of forests, but this effect is masked in the Petrolia gage data by variation in the weather. However, when minimum flows are adjusted by the July 15 flows, in order to account for wet or dry springs, the response to the logging shows up clearly (Figure 1).
With our annual summer drought, summer streamflow comes from water that percolates deep into hills, and trees affect deep percolation in several ways. First, trees roots penetrate deeply into the ground, drying soil at depth.
Second, trees intercept rain or snow before it hits the ground, and some of this evaporates directly from the surface of the canopy during and after rains, so that less reaches the ground. This “interception loss” varies, being greater as a percentage in light rains, but the scientific literature suggests 15 to 20% overall is a reasonable estimate. Interception by grass is much less, so trees reduce the effective rainfall. As the forest expands, interception increases as well.
Finally, the young fir forest that developed after the logging boom may be reaching peak water use. As a stand matures, water use by individual trees increases because they are bigger, but at some point use decreases because the trees are fewer and less effective at moving water to their needles. Taller trees need to pull harder to lift water into their canopies, which takes water-transporting cells with thicker walls and smaller passages in the sapwood, twigs, and needles.
Will aggressively thinning the forest help? In the very short and long run, probably yes, but in the medium term, the remaining trees probably will expand their root systems to capture the newly available water. How much thinning may help, and the time scales involved, are uncertain. We are hoping to set up experimental plots to collect data on the response of water use to thinning.