Question

6 Long Island East of New York City has 2.9 million residents on its 1200 mi². They use 120 gpd/capita. Annual recharge G ~ 12” from 40” total annual rain

a Estimate the average withdrawal rate, inches/month

b Give two reasons why Long Island is probably close to both its hydrologic and water quality safe yield

Answer 1

solution:

a.

• population=2.9*106
• water use by each capita= 120gpd =120*0.137ft3 /d=16.44ft3 ​​​​​​/d
• total water used in one day = 2.9*106*16.44ft3/d=47.676*106ft3/d
• total area = 1200mi2=3.3454*1010ft2
• withdrawal rate =47.676*106/(3.3454*1010)=0.001425ft/day
• withdrawal rate in inch/month= 0.001425*12*30=0.513 in/month.

b

1. Long Island is probably close to both its hydrologic and water quality safe yield because withdrawal rate is only 0.513in/month but recharge rate is 1inch/ month.
2. another thing is surface runoff is 40in/annum which leads to evaporation and continuously rainfall in expected month i.e. balence in hydrologio conditions will be maintained.