The Delaware River just rolls along, moving south from the site of the Montague gage. Photo by Meg McGuire

A drought near the river doesn't
mean a drought on the river

 

Droughts come and, luckily, mostly go. But when you're in the middle of one, no meteorologist would dare predict its end.

At the moment (and it can be a moment-to-moment thing — right now there's a fairly vigorous thunderstorm rattling my windows and blinking my lights) three of the four Delaware River states have varying degrees of drought watches and warnings, specific to each state and to counties in those states.

A map showing drought status in the Delaware River Basin as of Aug. 2, 2016. Click on the map to go to the full pdf. Map from DRBC.NETBut there's no drought for the Delaware River, which flows though several of the New York, New Jersey and Pennsylvania counties that are so assessed. It's just "rolling along" much like that Old Man River.(How the river's flow is kept steady is the subject of a previous story.)

Most of the states' stages of drought are assessed with a plethora of data: precipitation levels, reservoir/lake levels, stream flow, groundwater levels and even complicated models for ascertaining how moist the soil is.

Each state gathers its own statistics, analyzes its own data and comes up with its own conclusions.

For the river, that job belongs to the Delaware River Basin Commission. And while no job is easy, calling a drought for the river is tied to a simple metric: How much water is there in the three reservoirs of the New York City water supply that feed the Delaware River: Cannonsville, Pepacton and Neversink.

Click here to see a graph that explains simply and clearly how the process works.

There are five lines on the graph. Pay attention to the two top lines. One is a historical compilation of how full the reservoirs were on any given date.

The purple line is how full those reservoirs are "today," the dates run along the bottom.

Further down the graph there are another three lines. As soon as that "today" line crosses the top blue line, we transition from DRBC "normal" to "drought watch" operations, and a series of interconnected actions take place to conserve water and prevent that line dipping further down into drought warning.

While the designation of early drought status is relatively straightforward, what happens afterward can get complicated.

The NYC reservoirs are called on to release water, but care needs to be taken that filling the Delaware River water tank doesn't deplete New York City's water supply. Flow targets are closely monitored. The allocation for New Jersey's supply of Delaware River water out of the basin can be reduced. All sorts of reservoirs in the Delaware's watershed can come into play. If the drought gets serious enough. For example, a New Jersey reservoir built and owned by electric utilities for water-hungry power stations can release water into the Delaware to make up for the very large quantities of water they use that evaporates during power generation. This allows those utilities to maintain operation. A good thing since if there's a drought, it's usually hot and when it's hot, demand for power for air conditioning is strong.

To view a full-size version of this map, click on it. Map from DRBC.NETAt left is another graphic, a map of the Delaware River basin and all the reservoirs that can come into play when a drought emergency is declared by the DRBC.

Mind-boggling!

While the states and the Delaware River Basin Commission can sometimes get just a little bit bogged down in red tape and negotiations, the declaration of the early stages of drought are triggered by just that purple line descending below normal.

Each stage of the drought system carries with it automatic actions to respond to the drought — if the drought hits the last stage — an actual drought — a unanimous vote of the DRBC is required to declare an emergency before more drastic actions can be taken.

And since the Delaware is 330 miles long there's one other drought situation — a drought that affects the Lower Delaware and not the Upper. The river has another important monitoring point  - the flow gage at Trenton. When that registers below normal, that, will call for its own sets of actions.

For the DRBC, there's another important consideration when there's a chance the river will get low — it starts to lose its ability to repel the salt water that rushes into the basin and up the river twice a day with the tides. Salt water could create havoc with the municipal water systems that draw from the tidal Delaware — that's below Trenton — like Philadelphia, Pa. and Camden, N.J.

The salt line, as it's called, moves with the tides, but generally stays near Wilmington, Del. Back at the drought of record in the 1960s, the salt line moved up to the Ben Franklin Bridge in Philadelphia, and really close to the Philadelphia Water systems intake on the Delaware River.

To view a full-size version of this map, click on it. Map from DRBC.NETOh, I think we need another map!

So the key for possible drought actions is the amount of water in the reservoirs. Adam Bosch, the director of public affairs for New York City Department of Environmental Protection, Bureau of Water Supply, explained that there's a difference between a meteorological drought and a reservoir drought.

Reservoirs are about storage and probability of refill — in fact, that's what they were built for: To store water.

Meteorological droughts are basically about rainfall. Rain may fall in one area of the watershed and not another. Bosch referenced a convective thunderstorm that brought about 7 1/2 inches of rain to the Roundout Reservoir, not one of the Delaware River reservoirs. But because it was able to fill up, the NYDEP essentially turned off — temporarily — its use of the Delaware River reservoirs. And right now all the reservoirs are looking pretty good.

A map showing showing current reservoir levels. To view a full-size version, click on it. Map from nyc.govYet another map, please! Here are some links that might interest you if you're a drought geek:

New York

Pennsylvania

New Jersey

I'm not anti-Delaware (the state), but at this writing Delaware has no drought.

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