Basic
Hydrology
Hydrology, water cycle or hydrologic cycle
• Hydrology – the study of water; the science that encompasses the
occurrence, distribution, movement and properties of the waters of the
earth and their relationship with the environment within each phase of the
hydrologic cycle.
• Water cycle, or hydrologic cycle, is a continuous process by which water is
purified by evaporation and transported from the earth's surface (including
the oceans) to the atmosphere and back to the land and oceans.
• Hydrologic cycle - All of the physical, chemical and biological processes
involving water as it travels its various paths in the atmosphere, over and
beneath the earth's surface and through growing plants
HYDROLOGY AND WATER BALANCE
The science of hydrology describes and predicts,
occurrence, circulation and distribution of earths water.
Global Hydrologic Cycle:
• Transfer of water between land, ocean and
atmosphere
Land Phase Hydrologic Cycle:
• Movement of water on and under land surface
• Physical and chemical interactions with earth
materials and biological processes affecting
movement.
• (Davie, 2008)
Watershed and Forest Hydrology (FAO 2013)
• In the study of hydrology the spatial
unit we are pertaining to and mostly
concerned with are the catchment or
river basin or watersheds.
• Watersheds are area of land form
that is topographically delineated
and where water flows towards a
river system and ultimately ends in
the sea.
THE HYDROLOGIC CYCLE
EVAPOTRANSPIRATION
from surface bodies and
transpiration from
vegetation
Subwatersheds
Divides of
subwatershed
and
tributaries
Divide of
main
drainage
basin
The Importance of Hydrology
Human systems, communities and economies are
shaped and dependent on the availability and access
to water resources for various uses:
•
•
•
•
•
Household
Agriculture and Forestry
Manufacturing
Mining
Recreation
Water as the universal solvent dissolves more substances
than any other liquid. Wherever water goes, it takes along
dissolved substances from land surfaces where they come
from.
Attributes of Water
1. Quantity—the amount of water available for humans and
the environment.
2. Quality—the physical, biologically or chemically-influenced
usefulness of water for different purposes.
3. Timing—including floods and low flows.
4. Location—moving water to where we need it or storing it
in a strategic location.
Precipitation (Rainfall) –Major source of water
• About two-thirds of the precipitation that reaches the land surface return to the
atmosphere by evaporation from water surfaces, soil, and vegetation, and through the
plant transpiration.
• The remainder of the precipitation returns ultimately to the ocean through surface or
underground channels.
a) Groundwater and Surface water
b) Surface water consists of freshwater that flows and collects in rivers, lakes, or
reservoirs.
c) Groundwater are stored in underground rocks, known as aquifers, and renewed
through time by rain percolation.
d) Groundwater cannot be easily recharged (or not at all), once it is depleted.
e) Water sustainability from aquifers depends on proper management - available water
supply will not be withdrawn above the replenishment levels.
Thematic Problems with Water
Hydrology applies scientific knowledge and
mathematical principles to solve water-related
problems in society:
• Problems of quantity - supply from both surface
and groundwater
• Problems of quality – potability and safety for
certain use
• Problems of availability – access by various users
Change in Water Quality
• Water bodies provide environmental service using them as
receptacle of wastes as a non-consumptive use.
• This leads to the deterioration of the water quality, which further
aggravates the availability of water for consumptive purposes.
• Main problems affecting the quality of water resources in the
Philippines:
✓ groundwater -- salt water intrusion, leachate from wastes
✓ surface water -- pollution, siltation, and sedimentation.
Comparison of
surface and
groundwater
HOW DO WE
ESTIMATE
WATER
RESOURCES IN A
WATERSHED?
Data from the Department of
Environment and Natural Resources
showed that around 6.8 million
hectares of watershed areas were
assessed as vulnerable to
deforestation, biodiversity loss,
erosion, floods, landslides and water
pollution, while 14.2 million hectares
were identified as critical for the
national irrigation system.
NEDA. Environmental and Natural Resources
Accounting
The Water Balance
Method
Total Annual Rainfall = Actual
Evapotranspiration + Runoff + Ground
Water Recharge (GWR)
Annual Water Surplus = Runoff + GWR
Units in MCM = million cubic meters or
Units in mm
Surface Water A basin or watershed is the area tributary to a given point on a
stream and it is separated from adjacent basins by a divide or
ridge, which can be traced on topographic maps.
The following components require measurement:
a) streamflow
b) precipitation
c) evaporation
d) soil moisture
e) groundwater
Measuring Runoff Water or Discharge
Q= AV
where A = area and V = velocity
Source : ERDB
✓ With an average annual rainfall of 2,400 mm, the
mean annual run-off is estimated at about 257,000
million cubic meters (MCM), ninety percent (90%) of
the time (Concepcion, 2004).
✓ The National Water Resources Board (NWRB) reports
that the dependable surface water supply is estimated
at 206,230 MCM per year (MCM/yr) and 125,790
MCM/yr, for probabilities 50% and 80%, respectively
(NWRB, 2006).
Recharge –
The water balance computations presented in the IDRC- UPNHRC (1993) study show that only about 5.8 percent of the
rainfall recharges the aquifer.
In the case of other regions, the estimated inflow was based on
the assumption that 10 percent of rainfall recharged the
aquifers, but 50 percent of this mixed with salt water (NEDA,
1981).
Average annual
precipitation in
the Philippines
Climatic Water Balance Methodology - The basin
water balance defines the limits of the water resources of the
basin and forms the basis for allocating available resources to
different competing users.
P = AE + RO + GWR
Where:
P = precipitation or rainfall,
AE = actual evapotranspiration,
RO = total surface run-off,
GWR = net groundwater recharge
All the variables are in millimetre (mm) or expressed in cubic
meters
For purposes of the Water
Balance Study and the
management of DRB, the
Davao river basin with its 78
sub-watersheds has been
divided into 8 sub-basins.
Example: DRB Mean Annual Rainfall in mm
Values for the DRB Climatic Water Balance Study
P = AE + RO + GWR
Where:
P =Mean annual rainfall (MAR) of DRB, mean annual rainfall of each
sub-watershed)
AE =average of 30% of MAR
GWR = 2% of mean annual rainfall
Summary of Annual Water Balance for Davao River Basin and Sub-basins at Existing Conditions
Name
Area, km2
Unit
Rainfall [RF]
Actual Evapotrans-
Total Runoff [RO]
piration [AE]
Davao River Basin
1772.8
Davao Sub-Basin 1
243.3
Davao Sub-Basin 2A
270.7
Davao Sub-Basin 2B
99.5
Davao Sub-Basin 3A
234.2
Davao Sub-Basin 3B
261.3
Davao Sub-Basin 4A
310.7
Davao Sub-Basin 4B
139.2
Davao Sub-Basin 4C
229.1
Net GW Recharge
[GWR]
mm
MCM
2652
4702
781
1384
1821
3228
51
90
mm
2491
798
1641
52
MCM
mm
MCM
mm
MCM
mm
MCM
mm
MCM
mm
MCM
mm
MCM
mm
MCM
606
2602
704
1868
186
3196
748
1834
479
3467
1077
3467
483
1946
446
194
788
213
776
77
774
181
779
204
771
240
791
110
759
174
399
1762
477
1062
106
2362
553
1027
268
2630
817
2599
362
1156
265
13
52
14
30
3
59
14
28
7
65
20
77
11
30
7
Summary of Annual Water Balance for DRB Sub-basins at Existing Conditions in MCM
% of Mean
Annual
Rainfall
Net GW
Recharge
[GWR]
% of Mean
Annual
Rainfall
399
65%
13
2%
213
477
68%
14
2.0%
186
77
106
57%
3
1.6%
MCM
748
181
553
74%
14
1.9%
30,100
MCM
479
204
268
56%
7
1.5%%
Davao Sub-Basin 4A
27,798
MCM
1077
240
817
76%
20
1.8%
Davao Sub-Basin 5A
13,861
MCM
483
110
362
75%
11
2.2%
Davao Sub-Basin 5B
20,702
MCM
446
174
265
59%
7
1.6%
DRB/Sub-Basins
Area
(Ha)
Unit =
Million Cu
M
Rainfall [RF]
Davao Sub-Basin 1
24,458
MCM
606
194
Davao Sub-Basin 2A
27,459
MCM
704
Davao Sub-Basin 2B
8,184
MCM
Davao Sub-Basin 3A
23,398
Davao Sub-Basin 3B
Actual Evapotrans- Total Runoff
piration [AE]
[RO]
MCM = million cubic meters
3,320 MCM
ANNUAL WATER
SURPLUS IN DRB
=
3,230 MCM
TOTAL RUNOFF
+
90 MCM NET
GROUNDWATER
RECHARGE
2% of Mean Annual Rainfall
69% of Mean Annual Rainfall
Surface water is more abundant than groundwater stock
Estimates of Actual Evapotranpiration and
Ground Water Recharge
7.1 Average estimate of actual
evapotranspiration (Philippines)
(from J Perino; Roxas 2013)
7.2 Range of % groundwater
recharge from precipitation (various
studies)
At least 28-30% of total average
annual precipitation (mean annual
rainfall (MAR)
0-10% of total precipitation or
mean annual rainfall (MAR)
Conversions
1 mm= 0.001 m
1 liter = 0.001 cubic meter
1 metric ton= 1000 kg
1 ha= 10,000 square m
1 km= 1000 m
Exercise on the Basic Procedures and Data
Requirements – Estimating Available Water
Resources in a Watershed
1. Gather or estimate required data –
• area of each sub-watershed in each watershed
• Average of 5-10 year total annual precipitation (rainfall in mm) per subwatershed for each watershed in URB – use the estimates provided by
DOST or data from PAG-ASA weather stations or other weather stations
2. Get estimates from previous hydrology study (this is derived data)
or national average estimates for the following:
• Actual evapotranspiration data (percent of mean annual rainfall or MAR)
• Net ground water recharge (percent of mean annual rainfall or MAR)
3. Estimate the total available water – run off and ground water
recharge – of the sub-watershed and for the whole watershed or
URB