IJERD – International Journal of Environmental and Rural Development (2012) 3-2

Research article
erd

Evaluation of On-Farm Pond Water Budget in Khon KaenProvince, Thailand
KEWAREE PHOLKERN*
Groundwater Research Center, Faculty of Technology, Khon Kaen University, Kohn Kaen,Thailand
Email: rekawee@hotmail.com

VICHAI SRIBOONLUE
Groundwater Research Center, Faculty of Technology, Khon Kaen University, Kohn Kaen,
ThailandReceived 19 December 2011

Accepted 7 February 2012(*: Corresponding Author)

Abstract An on-farm pond is an essential technique for water resources management innortheast Thailand. It can be used for many purposes e.g. farm water supply,supplementary irrigation, livestock, aquaculture, and flood mitigation. The purpose of thisresearch is to evaluate the suitability of two calculation methods for on-farm pond waterbudget. Two on-farm ponds in Ban Wangwa were selected to be the study sites and thewater balance of the ponds including water inflow and water outflow components werecalculated. Water inflow was determined by two methods, namely watershed routing andsynthetic unit hydrograph. The results show that the synthetic unit hydrograph method
gives the better agreement to the observed data than the watershed routing method in bothof peak discharge and runoff volume. The main causes of water losses are evaporation,water usage, and seepage. Evaporation from the ponds was determined by two methods,
namely the Penman method using Priestley-Taylor evaporation equation and theEvaporation pan method. Subsequently coefficient of the Priestley-Taylor evaporation (α)  and coefficient of the evaporation pans (Kp) are 1.66 and 0.88, respectively. Theevaporation by pan evaporation is fast, simple and easier than Priestley-Taylorevaporation.
Keywords on-farm pond, water budget, inflow, evaporation

INTRODUCTION
An on-farm pond is an appropriate technology for water harvesting in northeast Thailand. The undulating landscapes, which dominate the northeast topography, enhance the efficiency and usabilityof on-farm ponds, by providing catchment areas and reasonable head for water conveyance. It canbe used for many purposes e.g. farm water supply, supplementary irrigation, live stock water consumption, aquaculture, and even flood mitigation (Suresh, 2002; Ngigi et al., 2005; Yoo and Boyd,1994; and Kumar, 1992).
The climate of the northeast is monsoon, where wet season is from May to October and dry
season is the rest of the year. Therefore more than 80% of annual rainfall is in the wet season, onfarm pond harvests water during 6 months of the wet season and supplies water for the next dry 6months. The study of the nature of the on-farm pond inflow and outflow is important for the onfarm pond design, construction, and management. To utilize an on-farm pond to its highest
potential and in an environmentally sound way, its study of water balance must be performed. Thisstudy presents a suitable method for calculating the pond water inflow and water loss of the onfarm ponds.

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STUDY AREA
The study areas located at two on-farm ponds in Ban Wangwa, 25 km south of Khon Kaen, werechosen as study sites. The two ponds are closed together, and one is considered to be the northpond while the other is considered the south pond. The north pond has the dimensions of 20304m. Its elevation of the bottom is 194.44 m above the mean sea level. The south pond has the size of17334 m and the elevation of the bottom is 194.95 m amsl. Fig. 1 shows the satellite image ofthe ponds and their environment which consists of upland crops, fallows, and eucalyptus woodlands.
The surrounding topography of the two ponds was surveyed using a level and a hand-held GPS.
The topographic contours and the watershed areas of the two ponds are shown in Fig. 2. The
catchment of the north pond is like a butterfly in shape which should produce a sharp peak with ashort time base. The catchment of the south pond is an elongated shape that should produce a mildpeak with a long time base. The outlet from the watershed or the inlet into the pond for each of thecases is quite special. For the north pond, the inlet is at the northeast corner of the pond and close to
the road, therefore we installed a rectangular weir for inflow measurement. For the south pond, theoutflow from the watershed passes through a ditch which leads across a road to the pond, a complicated inflow arrangement. The length and slope of the main channel for the north pond are 167 mand 0.0096 respectively, and for the south pond are 480 m and 0.0165 respectively. The areas ofthe watershed of the north and the south pond are 10643 m2 and 12618 m2 respectively. Piezometers were installed to observe groundwater levels, two for the north pond at P 2 and P4 in Fig. 2 andone for the south pond at P5. Automatic water level recorders were installed on north and southponds at P3 and P6 respectively. An automatic weather station was set up near the north pond. It
recorded rainfall, air and dew point temperatures, relative humidity, wind speed and direction, andnet radiation.
METHODOLOGY
Water balance equation or continuity equation can be used to describe the flow of water in and outof pond. Using all components that might possibly be significant, solve the continuity equation forgroundwater seepage (Chow, et al., 1988) as equation (1).
S
(1)
t
where P is precipitation,   Ri and  Ro are Runoff in and out respectively, Gi   and  Go are Groundwater
seepage additional to the pond and removal respectively, U is water use and S/t  is  change instorage.
Water balance in the on-farm pond was divided into two main sections. First section is theevaluation of accuracy of the water harvesting or water inflow of the on-farm ponds. The second
section was conducted to calculate the factors affecting water loss or outflow of the on-farm ponds.
P  Ri  Gi - E - R o - Go - U (1) On-farm ponds water harvesting
The water which flows into the pond consists of rainfall in to the pond, runoff and groundwater
inflow. The main component of water inflow is runoff. Two simple lump models were used in thisstudy, namely the watershed routing technique and the synthetic unit hydrograph method.
The watershed routing technique
Based on the assumption that the outflow from the watershed varies nonlinearly with the storage inthe watershed, van den Akker and Boomgaard (1996) suggested the following model.
q2 ⒸISERDk  0.5ttq1 ik  0.5tk  0.5t                                                                                                                            44(2)IJERD – International Journal of Environmental and Rural Development (2012) 3-2

where q1 and q2 are discharges as depth per unit time at time steps 1 and 2 respectively, i is intensity of excess rainfall, k is parameter and t is time interval.
The watershed routing model can be used to predict the flow rate from the present flow rateand intensity of rainfall data. The unit flow rate, q, can convert to total flow rate, Q, by multiplying
with the watershed area.

Fig. 1 Location of the two experimental ponds satellite image
LegendPosition ofinstrument
Contour line
Farm pond
Watershed areas

Fig. 2 Location watershed areas and its environment
The synthetic unit hydrograph
The unit hydrograph is a direct runoff hydrograph resulting from a unit rainfall (1 cm depth) of thespecific rainfall duration (Shaw, 1994). The unit hydrograph is normally derived from records ofrainfall and runoff data. When dealing with small watersheds, coupled rainfall and runoff data are
hardly available, we therefore resort to the synthetic unit hydrograph. The unit hydrograph that issynthesized from topographic and climatic features is called a synthetic unit hydrograph (Shaw,1994).
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The shape of a unit hydrograph may be assumed as the gamma function distribution (Aron andWhite, 1982). Akan and Houghtalen (2003) suggest an equation for coordinate of unit hydrographas equation (3).
uCp Atp[(t / t p ) exp(1  ( t / t p ))]n 1(3)where Cp = proportional constant, tp = time to peak discharge, A = watershed area, t = time, u =unit discharge at time t and n is a constant which related to Cp as equation (4).
n  1.0685 0.1175C p  0.782C p(4)

2

We selected 3 prominent storm events, two storms in the year 2006 on 30 Aug, and 17 Sept,and one storms in the year 2007 on 12 Sept for this study. The runoff data were interpreted fromthe incremental volume of the pond storage with the time interval of recording. Table 1 shows thedata of rainfall and runoff for the north and the south pond.
Table 1 The data of rainfall and runoff into the north and the south pond
30 Aug 2006
17 Sep 2006
12 Sep 2007
Time
Runoff (m3/s)
Runoff (m3/s)
Runoff (m3/s)
RainfallRainfallRainfall(min)(mm) north pond south pond (mm) north pond south pond (mm) north pond south pond
000000000010
0.20
0.005
000
0.031
00020
3.80
0.004
0.006
2.60
0.012
0.008
8.2
0.004
0.007
30
10.60
0.056
0.023
4.00
0.044
0.017
2.200.002
40
4.60
0.060
0.044
7.20
0.034
0.011
3
0.016
0.005
50
3.00
0.049
0.061
4.80
0.029
0.019
7.2
0.047
0.028
60
0.60
0.024
0.077
4.40
0.023
0.011
7
0.016
0.033
70
0.80
0.002
0.022
1.60
0.026
0.036
10.026
0.018
80
0.60
0.008
0.022
2.00
0.025
0.014
00.022
0.024
90
0.60
00.013
0.60
0.006
0.014
00.019
0.029
100
0.60
0.004
0.008
0.20
0.005
000.013
0.015
110
1.20
0.009
0.022
00.0010.008
00.0100.009
1201.60
0.013
0.004
0.20
0.001
0.006
00.008
0.004
130
0.60
0.002
00000.2
0.007
0.003
140
00.0080.008
00000.007
0(2) Water loss or outflow
The outflow of water or water loss from a pond, consists of evaporation, water use and groundwater seepage. The query found that both on-farm ponds are not water used for any activity. Groundwater seepage can be determined from the water balance equation or continuity equation. Thisstudy presents a method to determine water loss from evaporation, that water loss is the factor ofmost important (Vichai, 2008).
In the design and management of the reservoir, evaporative water loss is the factor of mostimportant (Vichai, 2008). The most popular method for determining the amount of evaporation,
Penman equation (1948) was employed. Penman equation combines the energy balance with themass transfer method and derives an equation to compute the evaporation from open water.
Penman equation  is quite difficult to be calculated and requires a lot of information. Then pond
evaporation data was estimated by Penman equation by using variables of other evaporation
equation that was more simple, as follows.
Priestley and Taylor evaporation equation
In 1972, Priestley and Taylor have revised the penman equation to be calculated easier as thePriestley and Taylor evaporation equation (5).
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E pond  


Er

(5)

where Epond = pond evaporation,  = gradient of the saturated vapor pressure curve at air temperature, Er = evaporation  rate,    = psychrometric constant.
  is  unknown parameter value. There will be slight variations in location of the study area.
Determining evaporation by Priestley and Taylor evaporation equation is much easier than Penman
equation.
Evaporation pan
The general methodology for evaporation calculation is the evaporation pan (Ep). There are various
types of evaporation pan. Adjustment factors or pan coefficients (K p) have been determined toconvert the data recorded in evaporation pans so that they correspond to evaporation from large
open water surface. Equation of the evaporation pan (Chow, et al., 1988) is as equation (6).
E pondK Ep(6)panThe evaporation from the pond was computed by using data from an automatic weather station,
which was set up on the study area in dry season of year 2006 and 2007 (November 2006 to April2007). Evaporation pan (Epan) data of study area from Thai Meteorological Department was used.
North pond 30-Aug-06

0.07

0.05

0.06

North pond 19-Sep-06

D a ta

DataWa t e rs h e d ro u t in g k =1.1

Watershed routing k=0.6

Un it h yd ro g ra p h C p =0 .3 7

0.04

Unit hydrograph Cp=0.65

0.04

0.02

0.01

00.02

0.00
010 20 30 40 50 60 70 80 90 100 110 120 130 1400Time (min)

(a)

(b)

South pond 30-Aug-06

0.07

Data

W a t e rs he d ro ut ing k=2 . 5
U nit hy d ro g ra p h C p =0 . 10 5

0.04

Q (m3/s)

Q (m3/s)

Q (m3/s)

0.04

0.03
0.02

0.02

Dat a

0.03

0.05
0.06

Time (min)South pond 12-Sep-07

0.04

Data
Watershed routing k=0.9
Unit hydrograph Cp=0.33

Unit hydrograph Cp=0.98

10 20 30 40 50 60 70 80 90 100 110 120 130 140(c)South pond 19-Sep-06

0.06

Watershed routing k=0.55

0.08

0.00

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Time (min)0.10

Un it h yd ro g ra p h C p =0 .9

0.06

0.02

0.01

Wa t e rs h e d ro u t in g k =1.3

Q (m3/s)

30.03

D a ta

0.10
0.08

0.03

0.04

Q (m /s)

Q (m3/s)

0.05

North pond 12-Sep-07

0.12

0.02

0.01

0.01

0.00

0.00
010 20 30 40 50 60 70 80 90 100 110 120 130 140

Time (min)

(d)

0.00

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Time (min)(e)

010 20 30 40 50 60 70 80 90 100 110 120 130 140

Time (min)(f)Fig. 3 Comparison of the predicted runoffs to the observed data, (a), (b), and (c)for the north pond and (d), (e), and (f) for the south pond
RESULT AND DISCUSSION
Three sets of rainfall-runoff data on 30 Aug and 19 Sept, 2006 and 12 Sept, 2007 were used. Theobserved hydrographs were plotted and compared with the two predicted methods in Fig. 2. It isclear from Fig. 3 that the synthetic unit hydrograph method has better agreement with the observedhydrograph than the watershed routing technique. For peak discharge, the synthetic unit hydroⒸISERD47

IJERD – International Journal of Environmental and Rural Development (2012) 3-2

graph method gives lower values that are closer to the observed data than the routing technique,although not by much. However, for the volume of flow, the watershed routing technique predictedmuch larger volumes than the observed data or the unit hydrograph method.
The k values vary in the ranges 0.6-1.3 and 0.55-2.5 for the north and the south pond
respectively. The coefficient Cp values of the synthetic unit hydrograph are in the ranges 0.37-0.90
and 0.10-0.98 for the north and the south pond respectively. The variations in both k and Cp
demonstrate the nonlinearity of the flow system. The average values of k of both ponds are rathervaried, but Cp of both ponds are fairly close. It is suggested that the runoff calculation from rainfall
for an on-farm pond water harvesting in northeast Thailand should be done by using the synthetic
unit hydrograph. The suitable Cp value can be obtained by fitting the model to several observed
data then averaging the values.
Evaporation of on-farm ponds, in November-February are in the ranges 4.48-4.77 mm/day,those in March and April are 5.26  mm/day  and  6.03  mm/day respectively as shown in Table 1. The
research results help to find the coefficients for the simpler method namely Priestley-Taylor
evaporation equation and the Evaporation pans method. The average coefficient  values ofPriestley-Taylor evaporation equation in Ban Wangwa are in the ranges 1.37-1.86 (average 1.67).
The pan coefficient Kp values of evaporation pans are in the ranges 0.80-0.91 (average 0.87) forBan Wangwa.
Table 2 Evaporation of water in the on-farm pond andthe most suitable values of  and Kp

November 2006

4.40

(Priestley and Taylorevaporation equation)
1.37

December 2007

4.34

1.82

0.90

January 2007

4.47

1.86

0.91

February 2007

4.80

1.80

0.85

March 2007

5.31

1.50

0.80

April 2007

6.04

1.40

0.85

4.89

1.67

0.87

MonthThe data of evaporation(mm/d)

Average

Kp(Evaporation pan)
0.89

CONCLUSION AND RECOMMENDATIONThe synthetic unit hydrograph method gives better predictions than the watershed routing techniquefor both the peak discharge and the runoff volume. The k-values of the routing technique vary in
the range 0.55-2.5 (avg 1.16). The Cp-values of the unit hydrograph method vary in the range 0.100.98 (avg 0.56). The variations in k and Cp demonstrate the nonlinearity of the flow systems.
Evaporation of on-farm ponds, in November-February are in the range 4.48-4.77 mm/day,those in March and April are 5.26  mm/day  and  6.03  mm/day respectively. The results help to findthe coefficients for the simpler method namely Priestley-Taylor evaporation equation and theEvaporation pans method. The average coefficient  value of Priestley-Taylor  evaporation equationis 1.66. The pan coefficient Kp value of Evaporation pans is 0.88. However, the evaporation by panevaporation is fast, simple and easier. Because the data used are less and easier to find.
ACKNOWLEDGEMENTS
This research is funded by Japan International Research Center for Agricultural Sciences (JIRCAS)
and Groundwater Research Center (GWRC), Khon Kaen University, Thailand. The authors thank Mr.
Suwanchai Nadee and Mr. Phayom Saraphirom for their useful suggestions.

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REFERENCES
Aron, G. and White, E. 1982. Fitting a gamma distribution over a synthetic unit hydrograph. Water Resources Bulletin, 18(1).
Chow, V.T., Maidment, D.R. and Mays, L.W. 1988. Applied Hydrology. McGraw-Hill.
Penman, H.L. 1948. Natural evaporation from open water, bare soil, and grass, Proc.R.Soc.London A, Vol.
193, 120-146.
Shaw, E.M. 1994. Hydrology in practice. 3rd ed., Chapman and Hall.
Sriboonlue, V. 2008. Hydrology. Faculty of Engineering, Khon Kaen University, Thailand.
Suresh, R. 2002. Soil and Water Conservation Engineering. 2nd ed., Standard Publishers.
Van den Akker, C. and Boomgaard, M.E. 1996. Hydrologie. Technische Universiteit Delft, Netherland.

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