Urbanisation

As familiar as this question may seem to us, only a few selected ones actually know the true answer to this puzzling question - how does urbanisation affect the hydrograph? First of all, in order to answer this question with precision, this has to be broken down into 2 parts, the water balance equation as well as the hydrograph.

Water Balance Equation: Ppt = Q + E + Change in S
Legend: Ppt = percipitation
E = evapotranspiration
Q = surface runoff
(delta S) = change in soil moisture

During urbanisation, there is a decrease in Evapotranspiration (E) due to the fact that trees are being cut down for the sake of development. As a result, this results in a decrease in soil moisture as well, as when trees are cut down, this reduces interception capabilities through throughfall and stemflow. As such normally, we would expect an increase in surface runoff (Q) by rainfall soil erosion. However, this is not the case as in urbanised areas, there is little soil exposed as soil is mainly covered up by concrete which is impermeable. As such, this reduces the infiltration capacity of soil greatly. As a direct result, as surface runoff = total ppt - rate of infiltration, there will be an increase in Q.



Now that we know that there is an increase in surface runoff, this would also lead us into discovering that there is basically a higher peak now in the hydrograph as well as a steeper rising limb as those 2 are dependant on amount of surface runoff generated as surface runoff is the fastest to enter the river. There will too be a lesser lag time as seen in graph above as basically, in urbanised areas, to protect against flooding as a result of high amounts of surface runoff, there are advanced drainage systems which will too speed up in the generating of surface runoff, decreasing lag time. As such, we now know more about effects of urbanisation on hydrographs. Oh yea. Great..

And ya, kudos to MOE for the graph above.

Joshua
JH40615