During periods of hot, dry conditions, both cool- and warm-season grasses can go dormant as a protective measure. If grass receives sufficient moisture, growth slows and blades remain green. History of DSOD inspection reports reveal a direct relationship to these factors: Example: - DSOD 2/2/2011 Inspection report "The usual green vegetation on the left side of the embankment is visible near the center of the picture. The area tends to dry out in the late summer".
First, where did the "dirt" come from?
Oroville has a seasonal weather pattern that is hot and dry in the summer and late summer times. However, one of the first questions that should be investigated is - "How did so much rich "favorable" growth soils occur in the large 164,648 sq foot area of the Green Wet region?" Was this through collection of fine trace soil particles from the upper Zone 3 hillside? There is evidence of wispy grass growth based sustaining material in the Zone 3 fill as seasonal rains reveal these wispy grasses on the full area of the slopes. Yet, rainfall will percolate downward into the Zone 3 fill by design (sands, gravels, cobbles, boulders). To form such a soil densified surface layer of 164,648 sq foot area of the Green Wet Area seems improbable for any "smart" rain "lateral" transport and focus to create such a large 164,648 sq ft "dirt spot" base. If you estimate the depth of this deposition, the cubic yards of material wouldn't add up from any upper Zone 3 migration scenario no matter how strange the rain could redirect its percolation back to focus on this area. (even at a fraction of an inch in depth over this sq foot area, the volume is significant [cubic yards]). Where did the growth medium come from? Has it been "ported" from the core region over these many years?
Back to the Grass…
When grasses reach a "Heat stress" condition, the response by the grass to these high temperatures, is to preserve itself and become dormant (i.e. turns brown). Some grasses are more resilient to heat stress as their photosynthesis processes are different and they thrive longer before becoming dormant. One example is the notorious pesky crabgrass. The key onset of heat stress-to-dormancy transition is 90 degrees Fahrenheit. One more step is required to "factor in" for the actual temperature conditions the grass may experience. That additional factor is from the sunlight thermal absorption in the Zone 3 materials. As the downstream dam face is southern, and the dam face is angled upward to the sun elevation angle, the efficiency of absorption of thermal energy is significant. The average daytime temperature in Oroville in July is 95F. Given a bleached light color of the Zone 3 fill cobbles, gravels, sand, boulders, a comparable energy absorption surface temperature increase would be similar to a concrete surface. Animal researchers, determining safety levels for pets, have tested the thermal rise on different surfaces during the hours of the day. This chart (below) is a very close match to the peak day temperature (95F) to Oroville's July peak day "average" temperature (95F). Using the concrete elevated surface temperature conversion, this would evidence that the Zone 3 surface would be at/near 125F. This is well above the 90F transition temperature that grasses trigger into self preservation "dormancy". If you use the "Grass in the Sun" conversion, the temperature is 105F. Still well above the 90F transition temperature into dormancy.
What about the Water?
The next factor to sustaining any greening, even with hardy warm-weather tolerant grasses, is the sustained water base. Yet, with Zone 3 fill surface temperatures at/near 125F, what happens to the evaporation rate of any upward and/or diagonal percolation of subsurface water? When you mix in the very arid typical summer conditions at Oroville, a 3 to 5 mph wind, and 125F surface temperature of the Zone 3 fill, the lbs per hour of water evaporation is significant. Depending on slight variations on these variables (such as wind speed), the evaporation rate could remove up to hundreds of gallons per minute for the full 164,648 sq foot Green Wet Area region. Certainly this moisture removal capacity would swiftly influence any grass growth transition into dormancy. The surface area of the Zone 3 fill would have the most intense effect of this moisture evacuation. Yet, there is no complete information as to "how" and "what" the mechanism is in how the moisture "reaches" the extreme lateral reach to the outside surface of the Zone 3 face. It could be possible that there is a plurality of "water flow" at different depths within the Zone 3 surface to where the outer layer is dissipated swifter compared to an inferred heavier "main" flow deeper below. So, in essence, the Greening is likely not a direct measure of the full seepage flow. The Greening is the result of a partial amount of the flow that moisturizes and wets the "growth medium". Seeking to extract a definitive assessment using Greening and Lake levels, are fraught with the more dominant effects of the "partial reach", the "strong evaporative losses" of the surface layer, and the "heat stress + aridity" that triggers Greening dormancy.
DSOD has already established that in "on-going drought conditions" that there is seepage present in the Wet Area. As this observation was done in an inspection with 81 days of zero precipitation, in the peak heat of Oroville avg (95F July 19) and the Inspection was done July 20, the presence of seepage at a 125F surface, with the evaporative power of removing up to hundreds of gallons per minute, is a testimony to what is really going on with an evidenced supported strong internal water source. (note: the grass was brown at the time of the inspection)
Oroville weather average temperature chart. Grasses transition to "dormant" (i.e. turn brown) at a threshold of 90F. Elevated Zone 3 fill temperatures in the peak summer are from 105F (grass surface) to 125F (Zone 3 surface) from direct sunlight thermal absorption.
Surface temperature graph (animal safety research - taking your pet for a walk) - sunlight on different surfaces - ambient temperature, grass temperature, Zone 3 equivalent absorption rate referenced to concrete surface.
