(48) 



The best estimate of the afflux in the Haarlemmermeer polder is 

 that by Mr. Elink vStekk '). This autlior bases his calculation on the 

 assumption tliat the \alne of rain minus evaporation, averaged over 

 many years, is practically the same for Rijnlajid and for the Haar- 

 lemmermeer polder. Rain is here tacitly assumed to be rain plus 

 surface condensation, and evaporation, evaporation plus the water 

 withdrawn hy plants. From the quantity of water discharued and 

 let in ovei' an average of 14 vears Mr. Elink Stekk then derives 

 with the aid of the assumption mentioned that the afflux in the 

 Haarlemmermeer polder is erpial to a quantity of water corresponding 

 to a height of 135 mm. -j- /v (A' being the afflux in Rijidand) over 

 the W'hoie surface. 



Now he puts /v=15 mm. i.e. \'i„ of the affiiix in the Haarlemmer- 

 meer polder which he calls an anqile estimate as 1 think it is; so 

 he finds for the amount of the afflux in the Haarlenunenncer polder 

 150 mm. 



The assumption mentioned that rain minus evaporation is ^equal 

 for Rijnhmd and for the Haarlemmennccr poidei- is not (piile correct 

 of course. The rain may be taken e(|ual, but not the evaporation. 

 The rainfall is in my opinion more regular on the average than is 

 indicated by our rain-gauges, ruder e(|ual meteorological conditions 

 the rate of evaporation depends i)rincipally on water for eAaporatiou 

 being or not being present. In the poldei's having a high summer- 

 level with regard to the laml which mostly consists of meadows, 

 evaporation ^vill be greater than in the Haarlemmermeer polder, 

 since water will always be present at the surface ; in the dunes on 

 the other hand it will be less. Considering the character of the grounds 

 in Rijnland we may assume that evaporation there will be slightly 

 greater than in the Haarlemmermeer polder. So if we apply to our 

 figure a correction L^, making it 150-[-^i, ^i ^^'dl be negative. 



^Ir. Elink Sterk has left out of consideration through lack of 

 data : 1. the quantity of w^ater admitted into the Groot Waterschap 

 van Woerden (having the same liosom as Rijnland) ; 2. The quantity 

 of water let in by locks into Rijnland and the Groot Waterschap 

 van Woerden. Calling these respectively L.^ and L^, the afflux in 

 the Haarlemmermeer polder is 



X; = 150 + A, + A, -f A3 mm. 



Now the quantities L^ and L^, are both small and certainly 

 positive; probably they are together smaller than L^. So if we omit 

 the three corrections Z-,, L„ and Z.,, the error can not be large and 



1) Verhandelingen van het Kon. instituut van ingenieurs. 1897 — 1898. p. 63 — 75. 



