PSSST..! It Could be the PST

PST stands for Practical Saturated Thickness - which differs from true saturated thickness.  It actually is pretty important in aquifers or aquifer sections that display a wide variability in storage coefficient and transmissivity.

According to the Kansas Geological Survey, who is doing a study on behalf of the Southwest Kansas GMD 3:

The PST considers only the net thickness of saturated sediments that significantly contribute to well yield from the water table down to the bedrock surface and differs from the saturated thickness (ST), which is the total thickness of saturated sediments between the water table and the bedrock surface. Thus, PST provides a more accurate picture of water availability and may also provide insight into future water-level trends at the scale of an individual well.

You may have 300 feet of saturated thickness in your well, but if half of that is comprised of tight sands, clays or other non transmitting materials, you're in for a surprise down the road.  SW GMD 3 is conducting a pilot study in the two areas shown in the attached graphic.

Drilling logs were used to estimate PST.  The logs used in this study are only from wells drilled into the bedrock surface. 207 logs were used for the Four Corners study area and 48 logs were used in the SE Stevens County area.  By evaluating the driller's logs and assigning aquifer parameters to the sections, it was determined that the pre-development PST fraction of ST was approximately 58% for both study areas.  The fact that only 58% of the saturated aquifer is contributing to well yields will likely impact future management decisions.

Saturated Thickness Variability

I'm often asked:  "What's the water table doing?"  It's a perfectly good question, but a difficult one to answer as succinctly as asked - especially in our area where the variability of conditions - from rainfall to actual measured declines - is so variable.  And averages only seem to confuse the issue, so I try to stay away from them. 

Case in point:  our water level elevation data from Thomas County - the heart of our GMD area.  The graph below shows four of the sixty-seven wells measured every Janauary to describe both the water table elevations and the saturated thickness.  (Click on the graph to enlarge it)  The wells are measured to the nearest 1/100th of a foot.  These 4 are the observation wells with:  the most saturated thickness (ST) in 1965; the least ST in 1965; showing the most decline (1965-2008); and showing the least decline (1965-2008).  I have also included (heavy black line) the annual average of all 67 wells.

From these 4 wells we start to see the variability within 1 County - not only in saturated thickness (from 62 feet to 175 feet) but in decline rates as well (from 5 feet to 38 feet).  And if you think spouting average values is misleading (or at least less than helpful) try answering with ranges of values.  Even less helpful to most.  And I've not even gotten into the variability over time, which finds our decline rates at any location changing from decade to decade - due mostly to longer term precipitation variability.

It wouldn't be so bad if it was just water level data, but it's other data as well.  The variability of our rainfall numbers for example stagger even the most hardened meteorologist.  So is the life of a groundwater manager.  Maybe I take it all too seriously.  Maybe people are asking me the rhetorical, ice-breaker question and really aren't that interested anyway.  And maybe that's why I keep all this stuff on our web page - so you can conclude your own answers. 

An Ogallala Groundwater Concept Rarely Discussed

Much has been and is being written about the Ogallala Aquifer and its groundwater situation.  Most articles do several things that I always want to question.  First, they tend to discuss a specific part of the Ogallala, but attach a headline that applies to the entire aquifer.

Take for example a recent Twitter post:  "Decline of the Ogallala Aquifer - a classic common pool resource problem."  When you follow the link and read the article, it's solely about the High Plains Texas portion of the aquifer, and moreover, the situtation (problem) is attributed almost exclusively to that states' groundwater management scheme - the Rule of Capture.  Yet, anyone skimming headlines only will come away thinking the entire Ogallala is a classic common pool resource problem - even though there are 8 different management approaches to the entire aquifer and only Texas uses the Rule of Capture.  I have probably 4 times over the past 6 months left comments to this effect on blogs and replied to twitter posts.  This will be a hard practice to correct, I'm afraid.

Secondly, a lot of the articles couch the problem of declines in a constant time frame - and predict a future-certain "plane crash".  For example, they say something to the effect that the water table is declining at 1.5 feet per year, and at this rate, 50% of the aquifer will be gone in 40 years and the aquifer will be dry in 80 years.  What is not commonly understood is that while a constant decline rate will predict the future, there will NOT be a constant decline rate because the wells producing the water and causing the decline will NOT be able to maintain a constant rate of production as the water table continues to drop. 

In fact, even if you are aware enough to know that reductions in saturated thickness will result in reduced pumping rates, most consider these reduced production rates as linear, and therefore predictable over time.  Not true.  A well's production rate decreases ever-faster as the saturated thickness dwindles.  In other words if you find a 10% reduction in your well yield with the last 20 feet of decline, the next 20 feet of decline will reduce your well yield significantly more than 10%, and so on.  The exact relationship is dictated by the aquifer characteristics so they vary from place to place - even within the same aquifer, but the relationship is a geometric one rather than arithmetic.

What this means is that the economic end of pumping will be closer than most think, AND the predicted future water level elevation of the aquifer will be higher than predicted as a result, AND an aquifer (in the sense it is being used in the article) will never be pumped dry.  Said another way, our "plane" will land softer than everyone is predicting.  However, of course, the core issue remains - any overdrafting will eventually see the plane land, while most would prefer that it keep flying forever (sustainability).  This, however, is an issue for a later blog.