Blog Entry #3

Jeju Island: HYDROLOGY

The Gotjawal Forest 

Dense Gotjawal Forest

The Gotjawal Forest covers approximately 86 square miles of Jeju Island. It is the direct water source for residents on the Island - providing for almot half a million people. The forest accumulates water through the diffusion of rainwater into the groundwater aquifer, this is permeable landform that water can be taken out of. Cracks in the basalt lava underneath the forest take in water and store it in layers underneath the forests surface. The area of storage where only water is stored within the pores is known as a saturated zone or the phreatic zone. There, water is stored within fragments of sediment beneath the aeration zone (vedos zone) layer. This layer is made up of both water and air directly underneath the Earth’s surface or in this case the lava beneath the Gotjawal Forest. 

Diagram 1 - Underground Aquifer

Drawdown & Recharge

Diagram 2 - Process of drawdown (water being extracted)

If the Island were to fall below the expected amount of rainfall throughout the year, the residents would suffer consequences from drawdown. Drawdown is the process of taking water out of the aquifers with the use of man made wells and pumping of the water. Without recharge, which is generally the process of refilling the aquifer with rainfall, the aquifer would stray from dynamic equilibrium which is when water being taken out is equal to the water being put in.

Compaction & Subsidence

Removal of water from an aquifer without recharge would eventually cause compaction which would lead to subsidence. Absence of water in the aquifer’s pores would cause the pieces of sediment to “compact.” As a result, the water table (level at which the water rises) would lower and the ground would follow (subside). The average amount of percipation in Jeju, as displayed below in row four of chart 1, shows that dynamic equilibrium should have been maintained in Jeju for many years because of constant monthly rainfall. The residents have yet to notice any signs of subsidence in the Gotjawal Forest and their water source remains reliable.

Climate data for Jeju-si, Jeju-do (1981−2010)
Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Average high °C (°F)	8.3 (46.9)	9.4 (48.9)	12.8 (55.0)	17.5 (63.5)	21.6 (70.9)	24.8 (76.6)	29.0 (84.2)	29.8 (85.6)	25.8 (78.4)	21.3 (70.3)	16.0 (60.8)	11.0 (51.8)	18.9 (66.0)
Average low °C (°F)	3.2 (37.8)	3.6 (38.5)	6.1 (43.0)	10.2 (50.4)	14.4 (57.9)	18.7 (65.7)	23.3 (73.9)	24.3 (75.7)	20.4 (68.7)	15.1 (59.2)	9.8 (49.6)	5.3 (41.5)	12.9 (55.2)
Precipitation mm (inches)	65.2 (2.567)	62.6 (2.465)	88.6 (3.488)	89.6 (3.528)	96.4 (3.795)	181.4 (7.142)	239.9 (9.445)	262.5 (10.335)	221.6 (8.724)	80.3 (3.161)	61.9 (2.437)	47.7 (1.878)	1,497.6 (58.961)
% humidity	65.3	64.9	64.9	66.5	70.4	76.8	78.3	76.5	73.7	66.9	65.1	65.1	69.6
Avg. precipitation days (≥ 0.1 mm)	12.6	10.3	11.2	10.0	10.4	11.8	12.5	13.5	10.8	7.0	9.3	10.8	130.2
Mean monthly sunshine hours	70.4	105.4	158.9	194.4	211.9	170.9	195.6	195.6	161.7	178.5	126.0	84.8	1,854.1
Source: Korea Meteorological Administration[11]

 Chart 1

Saturated forest surface after rainfall

Images taken from:

http://en.wikipedia.org/wiki/Jeju_Province

http://library.thinkquest.org/04oct/01590/humans/sources.html

http://en.wikipedia.org/wiki/File:Gotjawal_Forest.jpg

http://www.ramsar.org

http://dnr.wi.gov/org/water/dwg/OpCert/HTML/chapter3/wells4b.htm

Information taken from:

http://clasfaculty.ucdenver.edu/callen/1202/ 

http://clasfaculty.ucdenver.edu/callen/1202/Battle/Destroy/Hydrology/HydrologyBasics.html

http://en.wikipedia.org/wiki/Jeju_Province