Ef­fec­tive­ness of trees to re­duce flood­ing

Iran Daily - - Cultural Heritage & Environment -

Plant­ing trees can re­duce flood risk, but a high in­ten­sity for­est land use, such as graz­ing, can coun­ter­act the pos­i­tive ef­fect of the trees, a re­cently pub­lished study sug­gested.

As the fre­quency and sever­ity of flood­ing be­comes an in­creas­ing prob­lem, land man­agers are turn­ing to nat­u­ral flood man­age­ment mea­sures, such as tree plant­ing, to re­duce the risk, phys.org wrote.

When rain­fall ex­ceeds the rate at which wa­ter can en­ter the soil it flows rapidly over the land’s sur­face into streams and rivers. Trees can help to re­duce the risk of sur­face runoff by in­creas­ing the num­ber of large pores in the soil through which wa­ter can drain more eas­ily.

Land use, such as graz­ing, also af­fects the soil’s abil­ity to ab­sorb wa­ter; how­ever, while the ef­fect of land use on sur­face runoff has been well stud­ied in grass­lands, lit­tle is known about the ef­fect of land use in forests.

The study, un­der­taken by Lan­caster Uni­ver­sity and the Cen­ter for Ecol­ogy and Hy­drol­ogy and pub­lished in the jour­nal Geo­derma, in­ves­ti­gated the rate that wa­ter in­fil­trated the soil un­der trees at an ex­per­i­men­tal agro­forestry site in Scot­land.

Re­searchers found that in­fil­tra­tion rates were be­tween ten and a hun­dred times higher un­der trees, when the forested area re­mained rel­a­tively undis­turbed, com­pared with ad­ja­cent pas­ture. Where sheep were al­lowed to graze un­der the trees there was no ob­serv­able dif­fer­ence from the pas­ture.

They also com­pared for­est types — conifer for­est planted with Scots Pine and broadleaved for­est planted with sycamore — and found that in­fil­tra­tion rates were sig­nif­i­cantly higher un­der Scots Pine than un­der sycamore, but only when the for­est was un­grazed.

Us­ing rain­fall records the re­searchers were able to in­fer that a storm with a prob­a­bil­ity of oc­cur­ring at least ev­ery two years would be very likely to gen­er­ate sur­face runoff in the grazed for­est at the field site. How­ever, it was un­likely to oc­cur in the undis­turbed for­est ar­eas, re­gard­less of tree species, even dur­ing a 1 in 50 year storm.

Lead au­thor Dr. Kathy Chan­dler said, “Pre­vi­ous stud­ies have often com­pared largely undis­turbed forested ar­eas with land that is grazed or used to grow crops. This has led to the per­cep­tion that trees al­ways in­crease in­fil­tra­tion rates and, there­fore, re­duce the risk of sur­face runoff; how­ever, this study shows that for­est land use also plays an im­por­tant role.

“Tree plant­ing can make an im­por­tant con­tri­bu­tion to flood risk man­age­ment, but for­est buf­fer zones, with re­stricted ac­cess, strate­gi­cally placed to in­ter­cept sur­face runoff be­fore it reaches the stream may be more ef­fec­tive than larger scale plant­ing when the forested ar­eas are used for other pur­poses.”

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