Water Resources: Hydraulics and Hydrology
much runoff you're going to get, you can start drawing down your reservoir to prepare
it to catch the runoff.
Then, if everything worked perfect you would forecast how much runoff you were going
to get, and you'd draw the reservoir down and then the inflow that came in would exactly
fill it back up to the top of your irrigation pool by the beginning of the irrigation season,
if everything would be hunky dory. But what happens if you make a wrong forecast and
you say that there is going to be a lot more water coming off then actually comes off, then
you draw down too low and at the beginning of the irrigation season you don't have as
much water as you should have.
Where on the other hand if you make a forecast that is too low and you don't draw the
reservoir down far enough, then you can't take care of the flood and somebody gets
flooded because you didn't have enough storage to control it. So getting the exact balance,
forecasting that, is really a tough job. We used to work and work on forecast equations
based on what had happened in previous years, and you'd come up with regression
equations, taking into account all kinds of things like rainfall and temperature, antecedent
runoff, and other pertinent information.
You'd find equations that would maybe hit 95 percent of the time. But then there -would
be one year that nothing seemed to work on. It would be way off no matter what you did
with the general equation that you were coming up with. I believe, let's see, on the
Colorado River I think it was--I'm trying to remember what year it was. I believe it was
no, that wasn't it. It was after they had filled up the conservation storage in
Glen Canyon. Anyway, it was before I retired. Maybe it was `82, that sounds more like
it--maybe 82.
The forecast was way off because what happened is they got a real late snow in the year.
It was real late, like a real heavy snow which they hadn't predicted earlier, therefore they
hadn't drawn the reservoirs down very much or anywhere near as much as they should
have based on this later snow. Then it got real warm right after the snow, and they got
a lot of runoff. There was quite a bit of flooding and a lot of damage to some of the
Bureau projects from that big flood.
They had a tough time--they were very lucky that they didn't have even more damage
because of the fact that they hadn't been able to draw down, or didn't contemplate this,
or had no way of forecasting it. There was a lot of criticism about the whole thing on the
lower Colorado because there was all kinds of recreational facilities down there along the
river, and they were getting much higher releases than they had ever gotten before.
Of course, they had benefitted from many years where they were filling up the reservoirs.
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