Allan Frei

Please tell us what you do.

I’m a cli­ma­tol­o­gist and a pro­fes­sor in the Depart­ment of Geog­ra­phy at Hunter Col­lege. I ana­lyze data to see what hap­pened in the past, and then ana­lyze com­put­er sim­u­la­tions to pre­dict what might be hap­pen­ing now, and what could hap­pen if tem­per­a­tures get much warmer. How will cli­mate change affect the water cycle and there­fore the water sup­ply? Quan­ti­ta­tive work, like what I do, helps plan­ners who are look­ing at how dif­fer­ent extreme events can impact the infra­struc­ture and — may­be — how we should plan for more green infra­struc­ture in the future.

Sci­ence 101. So, can you give us some more back­ground about the his­to­ry of the water sys­tem in New York? 

Every­thing about the New York City water sys­tem is fas­ci­nat­ing – the his­to­ry of it, the social, polit­i­cal aspect of it; and the phys­i­cal infra­struc­ture of the sys­tem.

There is no fil­tra­tion on New York City water. That doesn’t mean it’s not treat­ed at all, but there is still no fil­tra­tion nec­es­sary. That’s very unusu­al for a large water sup­ply – among cities, we still have some of the best water qual­i­ty in the world. The rea­son the water is so clean is because in the region where most of New York City water comes from, the water­shed north of the city, nat­u­ral process­es main­tain a clean water sup­ply, from the time rain falls on the hills, col­lects in streams, and runs down to the reser­voirs.

Nat­u­ral fil­ters?

Nat­u­ral fil­ters, exact­ly. Only about 10% of the water comes from the part of the water sup­ply that is the old­est, and the near­est to New York City, in Westch­ester and Put­nam coun­ties – the Cro­ton reser­voir. That has more water qual­i­ty issues because it’s more dense­ly pop­u­lat­ed and the envi­ron­ment isn’t as pristine so there is a water fil­tra­tion plant being built to treat that part of the water.

Also, there is min­i­mal pump­ing of the New York City water flow. Water flows to New York City by grav­i­ty.

Through aque­ducts.

Yes. Big tun­nels, under­ground, that deliv­er the water from the reser­voirs upstate.

Is that what the sand­hogs do?

Exact­ly — the sand­hogs. They dig those and oth­er tun­nels that need to be built. The old­est part of the sys­tem, Cro­ton, start­ed deliv­er­ing water into New York City in 1842. The first water from the Catskills start­ed being deliv­ered in the 1920s, and the most recent of the main reser­voirs was com­plet­ed in the 1960s.

As the sys­tem was built, it cre­at­ed ten­sion between the city and the peo­ple in the rural, upstate water­shed com­mu­ni­ties. There cer­tain­ly used to be tremen­dous dis­trust of the city because the city need­ed a vast water sup­ply. And the city used polit­i­cal and eco­nom­ic clout to go up there and build reser­voirs. Entire towns were removed — because peo­ple built towns in val­leys, not up on top of the moun­tains. Val­leys, that’s where the reser­voirs end up. The local peo­ple cer­tain­ly didn’t have a voice – they tried to fight it may­be, but they couldn’t.

And peo­ple prob­a­bly did not get ade­quate­ly com­pen­sat­ed. Com­pen­sat­ed for their whole life — I mean, they had to move their whole life and move some­place else. For decades, there were peo­ple who thought that New York City had hid­den motives to do this. There were rumors that peo­ple in the city knew that there was oil in the ground, so they want­ed to move peo­ple off — rumors like that, to explain what was hap­pen­ing. I don’t know how bad it is now, but there was still fric­tion as recent­ly as the 1990s.

As a result of the Safe Drink­ing Water Act, which requires all munic­i­pal water sup­plies to fil­ter water unless grant­ed per­mis­sion to avoid fil­tra­tion, in 1989 the EPA told New York City to come up with a plan to pro­tect the water, or New York City would have to fil­ter the water. Which means: build fil­tra­tion plants. It’s bil­lions of dol­lars to build a fil­tra­tion plant, and hun­dreds of mil­lions of dol­lars a year to run and main­tain a plant. It’s a huge, huge endeav­or, so obvi­ous­ly New York City want­ed to avoid that at all costs. What fol­lowed was 8 years or so of inten­sive nego­ti­a­tions and dis­cus­sions between New York City, upstate res­i­dents, and oth­er stake­hold­ers.

In 1997 they arrived at a Mem­o­ran­dum of Agree­ment. New York City now pays for cer­tain things like sewage treat­ment plants for the local com­mu­ni­ties, and helps them with agri­cul­tur­al prac­tices that will reduce runoff, so the farm­ers are not incur­ring the costs them­selves, but they are pro­tect­ing the envi­ron­ment. Across the sys­tem, there are dif­fer­ent threats to the water qual­i­ty — it depends upon the land use. The west­ern part of the sys­tem, for exam­ple, has a lot of agri­cul­ture so there agri­cul­tur­al runoff is the biggest poten­tial threat.

So the solu­tion for clean water is coach­ing peo­ple and enforc­ing agree­ments?

Coach­ing peo­ple and work­ing with them to adopt meth­ods that pro­tect the water sys­tem, even the way they grow cer­tain plants around the fields or prac­tice irri­ga­tion. Part of the Mem­o­ran­dum of Agree­ment is that New York City buys land if there are crit­i­cal areas that are up for sale. They don’t take land by emi­nent domain, but if it’s up for sale, New York City will at least try to buy, if they decide it’s an impor­tant piece of land to pro­tect for water qual­i­ty.

That’s amaz­ing. And this was in 1997?

That agree­ment – and there have been a few adden­da to it, I think – but that agree­ment was signed in 1997. And that was a very, very tense time for peo­ple involved with this issue. You think of the­se as things in the past, but that was recent.

Did access to a sup­ply of clean water affect the suc­cess of NYC ear­ly on?

Very ear­ly on, the city was just the south­ern tip of Man­hat­tan. Over decades, there were a series of prob­lems because they just got ground water from wells and from ponds, and grad­u­al­ly those got con­t­a­m­i­nat­ed as the ear­ly city grew. And here wasn’t enough water to fight fires. Sev­er­al fires raged through the city in the 18th and ear­ly 19th cen­turies. They didn’t have the means to get the water in fast enough to fight fires, so they decid­ed they had to go north of the city to build some places where they could catch rain water. North of the city, at one time, was where 13th Street is now. [In 1830, the city built a stor­age tank on 13th Street and Broad­way, to sup­ply water for fire­fight­ing through 12” cast iron pipe laid in the ground.] 

Right — it’s good to keep that in mind.

Yes. Next came plans to go up to the Cro­ton sys­tem. In fact, they looked at dif­fer­ent rivers up there and end­ed up choos­ing the Cro­ton. Water start­ed com­ing in from the Cro­ton in 1842; that was a sig­nif­i­cant water sup­ply that would provide for growth. With­in a cou­ple of decades, the city looked again, far­ther north, and end­ed up in the Catskill Moun­tains, where now most of our water comes from – about 90% of the water for the city comes from there.

How have events like Hur­ri­cane Irene affect­ed the water sup­ply in New York?

We don’t know the answer yet, in terms of water qual­i­ty. But that’s cer­tain­ly an issue. The com­mu­ni­ties in the water­shed were affect­ed tremen­dous­ly.

Homes were destroyed.

Homes, roads, bridges, peo­ple died. Who knows what got washed into the reser­voir. Parts of sep­tic sys­tems – I mean, who knows what got into that. And in the east­ern part of the Catskills, there is cer­tain­ly a lot of ero­sion there, that’s the prob­lem under nor­mal con­di­tions. That sed­i­ment gets into the reser­voir and makes the water tur­bid.


Mud­dy, and under nor­mal con­di­tions, that’s an issue. So when­ev­er there’s a big storm, it’s a big issue. So when this hap­pened, small­er streams changed their course and jumped to dif­fer­ent places because things were erod­ing so fast.

And how do you antic­i­pate that the water sup­ply will be impact­ed in the future? I guess you don’t know the answer yet.

I don’t know the answer, you’re right. But it cer­tain­ly is a top­ic of con­cern not only for the New York City sup­ply, not just for the com­mu­ni­ties in the water­shed, but for oth­er com­mu­ni­ties in this region who, some of which get their water from the New York City sup­ply. The DEP [Depart­ment of Envi­ron­men­tal Pro­tec­tion] has put a sig­nif­i­cant amount of resources into a project to eval­u­ate the vul­ner­a­bil­i­ty of the New York City water sup­ply to cli­mate change. We’re in the mid­dle of that project now, so we don’t know the answers yet.

But there is some good news. In this region, cli­mate mod­els agree that there will be warm­ing — and that affects the water sup­ply because it impacts evap­o­ra­tion and tran­spi­ra­tion from trees. Peo­ple don’t real­ize what a big fac­tor that is – that’s a huge, huge fac­tor in the amount of water that stays in the sys­tem. So as sum­mers get longer and warmer, that rep­re­sents a sig­nif­i­cant drain of water from the sys­tem, because more water, what­ev­er falls, is going right back up into the atmos­phere — not stay­ing in the ground and in the reser­voirs. So just the warm­ing, that serves to make less water avail­able for human con­sump­tion and envi­ron­men­tal pur­pos­es – eco­log­i­cal ser­vices.

Is this good news?

No, that’s the bad news. The good news is that it seems like­ly, although it’s less cer­tain than the tem­per­a­ture pre­dic­tion, that there will be a mod­er­ate increase in pre­cip­i­ta­tion. That’s the oth­er side of the bud­get, right? Pre­cip­i­ta­tion comes in, and evap­o­ra­tion, or what we call evap­o­tran­spi­ra­tion, goes out. And what’s left is what we use in our dai­ly lives. And of course it’s what’s required in the creeks for the nat­u­ral land­scape. So, in terms of pre­cip­i­ta­tion, it’s expect­ed that there will be a mod­er­ate increase. There’s not com­plete agree­ment amongst the mod­els. There are some mod­els that are say­ing no, there will be less. But most mod­els are say­ing there will be some­where between five and fif­teen or twen­ty per­cent increase in pre­cip­i­ta­tion in this region.

That’s good news.

So then the ques­tion becomes, does pre­cip­i­ta­tion bal­ance out the loss­es, due to the warm­ing and faster evap­o­ra­tion? It’s very com­pli­cat­ed, for many rea­sons.  For exam­ple, you have the sea­son­al ques­tion: how warm is it going to be in a sea­son? Is it going to rain more in dif­fer­ent sea­sons? Rain in sum­mer, most of it ends up get­ting evap­o­rat­ed or tran­spired back into the atmos­phere.  On the oth­er hand, rain in spring and fall can end up stay­ing on the sur­face and end­ing up in the streams and reser­voirs of the water sys­tem. In the win­ter, some of that ends up as snow, which ends up stay­ing in the water sys­tem even more. It’s a com­pli­cat­ed ques­tion.

Just by com­mon sense, you think, well, there will prob­a­bly be less snow. More of the pre­cip­i­ta­tion will come as rain, because it’s going to be warmer, so the runoff sea­son, the hydro­log­i­cal cycle in the water­shed region, is going to change sig­nif­i­cant­ly. Every­one rec­og­nizes spring runoff. The streams are high in the spring. By the end of the cen­tu­ry that might change sig­nif­i­cant­ly. You may get a lot more runoff in the win­ter. There won’t be as much snow in win­ter; there will be more rain. The spring peak will still be a snow melt, but it won’t be as much. The­se changes will change the ecol­o­gy, and the econ­o­my, of the region, because com­mu­ni­ties upstate rely on ski­ing and fish­ing and vaca­tion­ing. The econ­o­my in most of the Catskills is not that strong. They rely on the­se things. So this poten­tial change is com­ing down the pipe.

But in terms of the amount of water com­ing into the sys­tem, it looks like the net result is that there’s going to be more water. That’s what we think now. So at least that is look­ing pos­i­tive.

I don’t think we have enough results to talk about how the­se changes are expect­ed to impact the water qual­i­ty. That becomes more com­pli­cat­ed. A lot of the­se things have to do with big events — in oth­er words, storms. Storms affect things, like ero­sion, which gets the water mud­dy. And water qual­i­ty also depends on bio­log­i­cal process­es that may be altered. The growth of phy­to­plank­ton and algae in the water can be dis­rupt­ed or changed. Reser­voirs or lakes may have sea­son­al changes in the tem­per­a­ture struc­ture and how they over­turn and mix in some sea­sons. That affects the bio­log­i­cal process­es as well. So how will all the­se things inter­act? There’s a group at DEP doing this, that I work with — they’ve got some good peo­ple work­ing on this. We just don’t have the answers yet.

How do you think New York could bet­ter adapt to the future? As a corol­lary to that, can res­i­dents of New York City do any­thing now to make the water sup­ply bet­ter?

Let me start with the first ques­tion, and actu­al­ly answer a ques­tion you asked ear­lier about Hur­ri­cane Irene. In all the­se issues, peo­ple try to address what the vul­ner­a­bil­i­ties are. I’m going to broad­en the dis­cus­sion to include oth­er com­mu­ni­ties. There has been an effort, for exam­ple, fund­ed by NYSERDA, to iden­ti­fy the vul­ner­a­bil­i­ties of New York State as a whole to cli­mate change. One of the sec­tors that we looked at was water. We look at the mod­els, and on the aver­age we won­der, will there be more water or will there be less water in dif­fer­ent regions? But it’s also the ques­tion of extreme events. It’s expect­ed in many parts of the world, includ­ing our parts of the world, that regard­less of whether pre­cip­i­ta­tion stays the same, or increas­es, or decreas­es, the fre­quen­cy of large events will increase. More big­ger storms. This is because in a warmer world, an expect­ed hydro­log­i­cal cycle would be accel­er­at­ed. You can get more extremes.

Peo­ple look­ing at this ques­tion recent­ly did think of this ques­tion of extremes, and now Hur­ri­cane Irene brings more atten­tion to the issue. And it wasn’t just Hur­ri­cane Irene, it was the con­text in which Hur­ri­cane Irene occurred that made this so dev­as­tat­ing to some regions. Because pri­or to Irene, the mon­th of August 2011 was already a huge mon­th for rain. In some places, record-set­ting. The ground was soaked. Usu­al­ly when it rains, some water soaks into the ground. But when the ground is sat­u­rat­ed, there’s no place to go but over the land sur­face and that’s what hap­pened with Irene — every­thing was so soaked before­hand, and then you had this record-set­ting hur­ri­cane and flood­ing. Then, a week lat­er, you had Trop­i­cal Storm Lee. It was Lee that did a lot of dam­age in the Catskills and oth­er areas that had record-set­ting pre­cip­i­ta­tion on top of what had already occurred. There were some places where the streams, after the trop­i­cal storm passed by, had real­ly record-break­ing flood­ing. Up and around Bing­ham­ton, New York. Record-smash­ing event.

So now, we have a good case study. We know it’s an unusu­al com­bi­na­tion, the amount of rain, then Irene, then Lee, but is it a one out of a hun­dred years that we get such a series of events? Or is it one out of every thou­sand years? How unusu­al is this event? If it was extreme­ly unusu­al in the past, is it going to go from once every thou­sand years and now hap­pen every hun­dred years?  Or would what used to be a once every hun­dred years event become a once every twen­ty or thir­ty year event? Fifty, six­ty, sev­en­ty, eighty years from now — how many should we expect? How does this event stack up in the past and in the future, and how can peo­ple pre­pare for that? What are the vul­ner­a­bil­i­ties for the­se sorts of events? Those are the impor­tant ques­tions we’re work­ing on.

How can New York adapt to an increase in water sup­ply?

I don’t know that I have the answer. In terms of how can we adapt, tech­ni­cal­ly, we have to make sure that our infra­struc­ture is robust, and it seems to have held up. Some of the infra­struc­ture, like the water sup­ply that deliv­ers water to New York City, held up through the storm — but there were many local infra­struc­ture exam­ples that failed. Bridges, local flood­ing, roads, tremen­dous num­ber of roads were under­cut because they were near creeks or rivers, and the cur­rent flowed so strong that they erod­ed the ground beneath the roads and the roads col­lapsed. Peo­ple were strand­ed for a long time. For days, some places up to a week or two. The­se are going to be more com­mon.

How can we main­tain the water qual­i­ty if the­se events are going to hap­pen?  That’s a tough ques­tion. We have to try to fig­ure out if we should expect more events like this. Expect the unex­pect­ed. One of the things about this cli­mate change issue is that many peo­ple say, what if the­se pre­dic­tions were wrong? And it’s not so bad? That’s true — but peo­ple don’t real­ize, what if the­se pre­dic­tions were wrong and it’s even worse than we’re pre­dict­ing?  No pre­dic­tions are going to be exact­ly right, so if we’re look­ing for a ball­park in the mod­els, some of them will under-pre­dict, and some of them will over-pre­dict.

There have been exam­ples of that already, not specif­i­cal­ly on the top­ic we’re talk­ing about, but for exam­ple, the Arc­tic sea ice. Sea ice in the Arc­tic has been dis­ap­pear­ing faster — way faster — than was pre­dict­ed ten years ago. They were say­ing, “Well, that may hap­pen; there could be ice-free sum­mers in the Arc­tic Ocean,” and we don’t know. May­be the last time that hap­pened was thou­sands of years ago. To the best of our knowl­edge. They were pre­dict­ing it might hap­pen by the end of the cen­tu­ry. Peo­ple are now say­ing the Arc­tic could be ice-free in sum­mer 10–20 years from now.

In terms of the water sup­ply in this region, we just have to be pre­pared for changes. We are wealthy enough, even though the coun­try is in an eco­nom­ic pick­le. We have resources enough here, unlike some oth­er places, so we can adapt more quick­ly. We can build the infra­struc­ture we need and fig­ure out how to han­dle the water qual­i­ty issue, as well as ris­ing sea lev­els. Which also actu­al­ly impacts the New York City water sup­ply in a round­about way.

How do you think City Atlas can be most use­ful to New York City res­i­dents? How can it edu­cate peo­ple about the water sys­tem that they are so lucky to have?

Again, I don’t know the answer, but that’s the impor­tant ques­tion. How can we use the Atlas, and any oth­er tools, to help peo­ple under­stand? Most peo­ple cer­tain­ly don’t know the his­to­ry of the water sup­ply, not even the recent his­to­ry. Peo­ple upstate in the com­mu­ni­ties know it bet­ter than peo­ple in the city. In the city, for most peo­ple all they know is you turn it on, and out it comes.

I think peo­ple have to be edu­cat­ed as to the his­to­ry and how this thing func­tions, because it’s not free. It’s not only not-free in terms of cost or resources, but it requires a lot of thought and knowl­edge. Peo­ple who work on the­se issues under­stand how the sys­tem works and try to under­stand how to keep it safe. Safe from los­ing our water sup­ply or water qual­i­ty. It’s a very dif­fi­cult job to do that. Per­haps aver­age cit­i­zens don’t appre­ci­ate it, and it’s worked because folks at the DEP, and oth­ers, have fig­ured out how to make it work over years.

One of the ideas that has sur­faced, a group of well known hydrol­o­gists wrote an arti­cle a few years ago say­ing some­thing that hydrol­o­gists under­stand: sta­tion­ar­i­ty is dead. In hydrol­o­gy, peo­ple would assume that what we’ve mea­sured his­tor­i­cal­ly — say, over the last fifty years — if you’re plan­ning for the future that it’s going to be approx­i­mate­ly the same. That sta­tion­ary mea­sure­ment is now dead. So how do we even func­tion now?

Peo­ple have to under­stand that just man­ag­ing the water sup­ply from day to day is a dif­fi­cult job. And now the entire sys­tem is going to be less pre­dictable. Even on calm days, deci­sions are made that are not that easy to make. For exam­ple, one of the things they do in one of the water sys­tems is let the water accu­mu­late on one side of a reser­voir and then flow over the wall to the oth­er side, so the sed­i­ment accu­mu­lates in the first side, and the clean water is on the oth­er side. They can con­trol the releas­es from both sides. But what hap­pens is, if you release into the rivers the sed­i­ment-laden water, the one with more sed­i­ment in it, that’s what ends up in the rivers down­stream of the reser­voir, so the local com­mu­ni­ties aren’t hap­py. [This kind of maneu­ver can be com­plex even in sta­ble weath­er.]

You’re jug­gling New York City’s needs, the local community’s needs, the ecosystem’s needs, and chang­ing weath­er pat­terns. It’s a real­ly hard job. May­be I’m biased, but…I’m not prais­ing myself. The guys who work at the DEP and man­age this water sys­tem, they do a real­ly good job. They real­ly do. They do a hard job. You hear that, guys?

About Allan Frei:

Dr. Frei, the Deputy Direc­tor of CISC, is an asso­ciate pro­fes­sor in the Geog­ra­phy Depart­ment at Hunter Col­lege, CUNY. After receiv­ing his Ph.D. from Depart­ment of Geog­ra­phy at Rut­gers Uni­ver­si­ty in 1997, he spent four years at the Nation­al Snow and Ice Data Cen­ter (NSIDC), which is part of the Coop­er­a­tive Insti­tute for Research in Envi­ron­men­tal Sci­ences (CIRES) at the Uni­ver­si­ty of Col­orado. In 2001 Dr. Frei moved to Hunter Col­lege. Dr. Frei is a cli­ma­tol­o­gist whose research inter­ests include issues relat­ed to cli­mate change, includ­ing links to snow cov­er and sea ice across the North­ern Hemi­sphere, as well as water resources in the New York City water­shed region.


Pho­to by Mau­reen Dren­nan
Graph­ic by Flo­ri­an Brozek


Read more: New York’s water sup­ply, and PlaNYC

An info­graph­ic of New York’s water from 1800 to the present