ESTIMATION OF AGRICULTRAL WATER DEMAND

IN GAZA STRIP / PALESTINE

 

 

 by

Said Ghabayen

CONTENTS

1                  Abstract

2                  Objectives

3                  Study Area

4                  Available Information

5                  Data Preparations

6                  Equations & Calculations

7                  Results & Discussion

8                  Uncertainties

9                  Future Work

10            References

 


 


1      ABSTRACT

 

Water is the most precious and valuable natural resource in the Middle East in general and in Gaza Strip in particular. It is vital for socio-economic growth and sustainability of the environment. Gaza Strip is in critical situation that requires immediate and concerted efforts to improve the water situation in the term of quality and quantity.  Demand greatly exceeds water supply.  In addition water quality is very poor and the aquifer is being over pumped.  Very limited water supplied for domestic use is potable.   About 70% of the total pumped water is used for agricultural purposes.

 

For sound planning, it is very essential to have accurate figures for current water consumptions. These figures are only accurate for municipal and industrial consumption as the related water sources are well monitored. Accurate figure for agricultural consumption does not exist. Currently, there are over 3,500 agricultural water wells of which 50 % are considered illegal. In addition to that, most of the wells are not metered or the meters are very old and deteriorated.

 

In this term project, agricultural water consumption was estimated by using Arc View GIS based approach and utilizing the available information of land use, rainfall, and other meteorological data.

 

The total annual crop water requirement is calculated to be 56.3 Mm3. knowing that the average annual well abstraction is 80 Mm3; the losses due to conveying system deficiency, irrigation techniques, and over application are estimated at 30 %. Maps showing the spatial distribution of the monthly crop water requirement are also produced.

 

 


2      OBJECTIVES

 


The main objective of this term project is to use Arc View GIS based approach to estimate the crop water requirement for Gaza Strip / Palestine.

                            

Another objective is to produce maps showing the special distribution of the monthly crop water requirement for the use of water resources planners.

 


3      STUDY AREA

 

Figure (1) shows a location map of the study area, Gaza Strip is one of the Palestinian Self Government area located at the southeastern edge of the Mediterranean. Using UTM projections the area is located in UTM zone 36 with 330E as the central meridian and the equator as the reference latitude. The numbers in the map shows how far is Gaza Strip from central meridian and the reference latitude.

 

Figure (1): Location map of the study area.

 

 

Gaza Strip if 40 km long and 7- 14 km wide with total area of 365 km2. The current population of Gaza Strip is about 1.1 million occupying half of this area; the rest of the area is used for agriculture as the main source of living.

 

Groundwater from the coastal aquifer is the only source of fresh water for the area, which is currently under serious stress due to mining, pollution from different sources and seawater intrusion.

 

Gaza Strip is characterized by semi-arid temperate climate; hot dry summer and cold rainy winter. The average long term annual rainfall is 350mm occurs between October and March. The long-term average annual open surface evaporation is 1300mm with its maximum in summer season (June August).

 

                             Figure (2) below shows the average monthly values for rainfall, evaporation and effective rainfall. The effective rainfall is the part of the rainfall used by the plant, calculated using USDA-SCS procedure, which will be explained later in this report.

 

 

Figure (2): Monthly variation of total rainfall, effective rainfall, and evaporation.

 

                                   

                              

4      AVAILABLE INFORMATION

 

        Shape files for current land use, crop cover, base map, political boundaries, and elevation contours (sources: Palestinian Ministry of Planning and International Cooperation).

 

        20 years rainfall records for 8 meteorological stations covering Gaza Strip (source: Palestinian Meteorological Department).

 

        Crop information such as growing season, crop height, and crop coefficient (source: FAO papers 24, 33, 56).

 

 


5      DATA PREPARATION

 

CLIMATE

DATA                       Data available from 8 meteorological stations distributed along Gaza Strip were collected. A new point shape file was created. The stations were located. New fields for average monthly rainfall for each station were added by editing the attribute table.

 

The reference evapotranspiration was calculated by excel spreadsheet using Penman Monteith FAO equation. New fields for monthly evapotranspiration for each station were then added by editing the attribute table.

 

Grid themes for monthly rainfall and evapotranspiration were created by loading the special analyst and using interpolate grid command from the surface menu. The grid cell size was chosen as 100 meters, as it is the average farm size in the area. Then these grid themes were clipped to the study area boundaries using ESRI avenue script clipping file.

 

Figure (3) below shows the annual rainfall distribution for Gaza Strip. Notice that the rainfall amount decreases to almost half as we move 40 km to the south. This is so because Gaza Strip is located in the transitional zone between a semi humid climate north of Gaza Strip and arid climate of Sinai desert of Egypt in the south. Values for evapotranspiration are shown in Table (3) after discussing the related calculations.

 

Figure (3): Annual rainfall distribution for Gaza Strip (mm/year).

           

 

 

CROP DATA        for the purpose of this project, the crop patterns were classified into five main categories. The classification was based on growing season, crop coefficients, and crop cover and height. Table (1) below shows these categories.

 

Table (1): Crop categories.

Crop Category

Crop Type
Citrus

Orange / lemon / grapefruit

Fruit trees

Apples / pears / peaches / apricots / almonds

Vegetables1

Cucumber / squash / cabbage

Vegetables2

Tomato / sweet peppers / egg plants / potato

Field crops

Wheat / barley

 

A new polygon theme was created for each crop category then all these themes were merged using geo-processing wizard. Crop coefficient values (Kc) for each crop categories were obtained from FAO papers 56 and 33 and then added to the attribute table of crop cover theme. Table (2) below shows the value of crop coefficient and the growing season for each crop categories.

 

Table (2): Crop Coefficient values (Kc) for different crop categories.

 

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Citrus

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.65

0.65

0.65

0.65

0.65

Fruit trees

0.9

0.9

0.9

0.65

0.65

0.65

0.65

0.4

0.4

0.4

0.4

0.9

Vegetables1

1.15

1.15

0.95

 

 

 

 

 

0.6

1

1

0.75

Vegetables2

0.8

 

 

0.6

0.6

1.15

1.15

0.8

0.6

1.15

1.15

0.8

Field crops

1.15

0.4

 

 

 

 

 

 

 

0.3

0.3

1.15

 

 

Grid themes for monthly crop coefficient were created and clipped in the same way discussed in the climate data section. The cell size was also chosen as 100 meters. Figure (4) below shows the crop distribution grid for the study area based on the above classifications.

 

Figure (4): crop cover distribution grid for Gaza Strip.

                                   

 

                                    At this stage, each grid cell contains values for rainfall, reference evapotranspiration, and crop coefficient, which are related to crop type and growing season. This was prepared for each month to enable the map calculator in Arc View to perform the necessary calculations as will be discussed in the next section.

 


6      EQUATIONS AND CALCULATIONS

 

REFERENCE

EVAPOTRANSPIRATION   

Reference evapotranspiration (ET0) was calculated using Penman Monteith FAO equation presented in FAO paper 56. This paper defines the reference evapotranspiration as the evapotranspiration from the hypothetical grass reference surface and provides a standard to which evapotranspiration in different periods of the year or in other region can be compared and to which the evapotranspiration from other crops can be related. Penman Monteith equation cab be written as:

                                   

---          ----(1)

 

 

                                    Where:

 

 

Rn      = net radiation at the crop surface                 (M J m-2day-1);

G       = soil heat flux density                                  (M J m-2day-1)

T       = mean daily air temperature at 2m height      (0C)

U2      = wind speed at 2m height                            (m s-1)

es       = saturation vapor pressure                          (kPa)

ea       = actual vapor pressure                                (kPa)

D       = slope of vapor pressure curve                   (kPa 0C-1)

g        = psychometric constant                              (kPa 0C-1)

 

                                   

These parameters are calculated using meteorological data such as altitude, latitude, mean relative humidity, sunshine hours, absolute minimum and maximum temperature, mean minimum and maximum temperature, mean monthly temperature, and wind speed. The followings are the equations used to calculate these parameters based on FAO paper 56.

 

        (Rn): The net radiation at the crop surface is given by the equation:

                                   

                                                                                                                        ---- (2)

Where

 

                                      Rns  = the net solar or short wave radiation given by:

                                   

                                                                                                                       ---- (3)

 

                                                Where

 

Rs  = the total solar or short wave radiation given by:

 

                                                                                                 ---- (4)

                                               

Where:

                              

n/N = relative sunshine hours determined by n which is the measured sunshine hours and N which is the mean daylight hours given for different latitudes. For Gaza Strip values of N are presented in Table (3).

 

Ra  = extra terrestrial radiation which is based on the latitude. Values of Ra for Gaza Strip are also shown in Table (3).

 

Referring back to equation (2):

 

Rnl  = the net long wave radiation given by:

 

     ----(5)

 

            Where:

 

Tmax = absolute monthly maximum temperature in Kelvin

 

Tmin = absolute monthly minimum temperature in Kelvin

 

Rso  = clear sky solar radiation given by:

 

                                                ----(6)

 

Where

 

Z  = the latitude

 

        (G): Soil heat flux  in (M J m-2 day-1) given by the equation:

 

                                       ----(7)

                                               

                                                Where:

 

Tmonth(i+1) = mean monthly air temperature for the month after

 

Tmonth(i-1) = mean monthly air temperature for the month before

 

        (P): Atmospheric pressure in (kPa) given by the equation:

 

                                            ----(8)

 

        (g): Psychometric constant in (kPa / 0C) given by the equation:

 

                                                                ----(9)

 

        (e0): Saturation vapor pressure in (kPa) given by the equation:

 

                                             ---(10)

     

Where:

 

T  = the mean monthly temperature in 0C

 

        (es): Mean saturation vapor pressure in ( kPa) given by the equation:

 

                                                               ---(11)

 

            Where:

 

= saturation vapor pressure at maximum temperature

 

= saturation vapor pressure at minimum temperature

 

        (ea): Actual vapor pressure in (kPa) given by the equation:

 

                                                ---(12)

 

            Where:

 

RHmean  = mean monthly relative humidity

 

 

        (D): Slope of saturation vapor pressure curve in (kPa / 0C) given by the equation:

 

                                                           ---(13)

 

        (U2): Wind speed corrected at 2 m above the ground surface in (m/s) given by the equation:

 

 

                                               ---(14)

 

Where:

 

UZ = the wind speed at given elevation

                            

                             The monthly values of all of the above parameters for Gaza Strip are shown in Table (3).

 

EFFECTIVE

RAINFALL            The effective rainfall is calculated using USDA-SCS (US Department of Agriculture Soil Conservation Service) procedure. This procedure is used for general estimates of monthly effective precipitation for planning and most systems design.  The USDA-SCS procedure is described by the following equation:

 

                                                                     ---(15)

 

 

                                    Where:

 

f = correction factor depends on average net application depth or soil moisture depletion before each irrigation. For the purpose of this study f value is taken as 1.0, which the value corresponds to the net depth of water depletion of 75mm assumed as the average value for the study area.

 

P  = the gross monthly rainfall in mm

 

ET0 = the monthly reference evapotranspiration

 

Figure (2) showed the calculated values of monthly effective rainfall for the study area.

 

Table (3): Calculation of reference evapotranspiration from meteorological data.

 

 

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

mean relative humidity (%)

RHmean

66

69

64

67

73

77

76

75

65

66

72

62

actual daily sunshine hours

n

4.75

5.54

6.9

9.49

7.81

9.93

10.7

10

9.8

9.2

6.8

4.5

mean day light hours

N

10.2

10.95

11.8

12.75

13.55

14

13.85

13.15

12.2

11.25

10.4

10

relative sunshine duration

n/N

0.47

0.51

0.58

0.74

0.58

0.71

0.77

0.76

0.80

0.82

0.65

0.45

absolute minimum temperature (C0)

Tmin

5

8.6

8

10.5

15.2

19.2

21.5

23.5

21.2

17

14.8

9.6

absolute maximum temperature (C0)

Tmax

23.5

26.7

28.5

40.4

36.8

29.8

32.8

33.2

32

39

27.5

31.4

mean minimum temperature (C0)

T'min

10.29

11.42

11.9

15.8

18.57

21.56

23.2

25

22.7

20.4

17.4

12.7

mean maximum temperature (C0)

T'max

18

18.38

19.6

24.74

24.78

27.12

29.8

31.9

30

27.6

24.2

20.8

mean monthly temperature  (C0)

Tmean

14.14

14.9

15.6

20.27

21.67

24.3

26.7

28.3

27

24

20.8

16.6

soil heat flux (M J m-2 day-1)

Gmonth

0.12

0.10

0.38

0.42

0.28

0.35

0.28

0.02

0.30

0.43

0.52

0.47

saturation vapor pressure (KPa)

e0

1.61

1.69

1.77

2.38

2.59

3.04

3.50

3.85

3.57

2.98

2.46

1.89

saturation vapor pressure at max temperature (kPa)

e0(Tmax)

2.06

2.11

2.28

3.12

3.13

3.59

4.19

4.73

4.24

3.69

3.02

2.46

saturation vapor pressure at min temperature (KPa)

e0(Tmin)

1.25

1.35

1.39

1.80

2.14

2.57

2.84

3.17

2.76

2.40

1.99

1.47

mean saturation vapor pressure (KPa)

es

1.66

1.73

1.84

2.46

2.63

3.08

3.52

3.95

3.50

3.04

2.50

1.96

actual vapor pressure (KPa)

ea

1.09

1.19

1.18

1.65

1.92

2.37

2.67

2.96

2.28

2.01

1.80

1.22

vapor pressure deficit (KPa)

es-ea

0.56

0.54

0.66

0.81

0.71

0.71

0.84

0.99

1.23

1.04

0.70

0.75

slope of saturation vapor pressure curve (KPa/C0)

D

0.10

0.11

0.11

0.15

0.16

0.18

0.21

0.22

0.21

0.18

0.15

0.12

measured wind speed (km/hr)

 

12.12

12.1

17.1

14.33

12

10.66

7

5

7.7

6.4

7

12

measured wind speed (m/s)

Uz

3.37

3.36

4.75

3.98

3.33

2.96

1.94

1.39

2.14

1.78

1.94

3.33

wind speed corrected for 2m altitude (m/s)

U2

2.27

2.27

3.21

2.69

2.25

2.00

1.31

0.94

1.45

1.20

1.31

2.25

extraterrestrial radiation (M J m-2 day-1)

Ra

20.5

25.3

31.05

36.65

40

41.3

40.65

37.95

33.1

27.1

21.65

19.15

solar radiation

Rs

9.90

12.73

16.84

22.80

21.53

24.97

25.86

23.92

21.57

17.86

12.49

9.10

clear sky solar radiation

Rso

15.38

18.99

23.30

27.50

30.02

30.99

30.50

28.48

24.84

20.34

16.25

14.37

 

Rs/Rso

0.64

0.67

0.72

0.83

0.72

0.81

0.85

0.84

0.87

0.88

0.77

0.63

net solar/short wave radiation

Rns

7.62

9.80

12.97

17.56

16.58

19.23

19.92

18.42

16.61

13.75

9.62

7.00

net long wave radiation (M J m-2 day-1)

Rnl

3.38

3.66

4.19

4.88

3.57

3.54

3.53

3.15

4.20

4.81

3.86

3.44

net radiation  (M J m-2 day-1)

Rn

4.24

6.14

8.77

12.68

13.01

15.69

16.39

15.26

12.41

8.94

5.76

3.56

Reference Evapotranspiration (mm/day)

ET0

1.99

2.30

3.28

4.30

4.15

4.84

5.20

5.05

4.49

3.20

2.06

2.10

 

 

CROP WATER

REQUIREMENT          

The crop water requirement is then calculated also using FAO pape-56 equation:

 

                                                                       ----(16)

 

 

Where:

 

Kc = the crop coefficient.

 

The map calculator is then used to calculate the value of crop water requirement for each grid cell using the crop coefficient, reference evapotranspiration, and rainfall grid value for each cell. In case the effective rainfall is more than the crop evapotranspiration the crop water requirement will be negative according to equation (16). In this case, map calculator is used to convert all the negative value cells to Zero. Basically in this case no irrigation is needed.

 


7      RESULTS AND DISCUSSIONS       

 

For each month, a map showing the spatial distribution of crop water requirement was produced. Figures (5) and (6) below show the monthly crop water requirement for January (rainy month) and July (dry month).

 

Figure (5): Crop water requirement for January (mm).

 

 

 

Figure (6): Crop water requirement for July (mm).

 

                                    As the Figures (5), (6) indicate, the crop water requirement for the month of January is limited to the southern area where the monthly rainfall is not sufficient for crop growing. In the month of July, when there is no rainfall and the reference evapotranspiration is at its highest value, much more irrigation is needed everywhere.

 

                             Figure (7) shows the spatial distribution of annual crop water requirement. This was prepared by using the map calculator to sum the monthly grid cell values for crop water requirement. The annual crop water requirement for Gaza Strip varies from 0 to 9000 m3/gridcell (the grid cell is 100m x 100m). The variation depends on cropping pattern growing season, and rainfall distribution. Zero value occurs in the area cultivated with rainfed crops. High values occur in the areas cultivated with citrus trees, which is characterized by high annual crop evapotranspiration.

                                   

                                    Figure (7): Annual crop water requirement distribution for Gaza Strip (mm).

 

Figure (8):Total monthly crop water requirements for Gaza Strip.

 

Figure (8) shows the monthly variation in crop water requirement for the whole area. Although high values generally occur during summer months the highest crop water requirement was found in October. This is so because in October all the land will be cultivated and the monthly rainfall is low. In other words October is the month where all the crops contribute to the weighted crop coefficient. The same could be said about November but the steep variation in the monthly rainfall causes the drop in the monthly crop water requirement.

                                                                                                             

The total crop water requirement for Gaza Strip is calculated as 56.3 million m3. If this figure is compared with the annual agricultural well abstraction which is estimated at 80 million m3, one can conclude that the total losses due to delivery system, partially traditional irrigation techniques, and over application by the farmers is about 30 %.

 


8      UNCERTAINITIES

Uncertainties are referred to the possible sources of errors, which occurred either during data preparation or calculations. The main sources of uncertainties involved in this term project are listed below:

        Generalizing crop coefficient for the crop category rather than applying it to each crop type. As discussed before, crops were classified into five main categories based on crop coefficient, growing season, and crop height. Some errors may be encountered due to the slight variation in cropping season and the crop coefficient for different stages of crop development within each crop category. But for planning purposes, especially when dealing with Macro level, these errors are considered minor.

 

        Another uncertainty could result from averaging the net depth of water depletion as 75mm over the whole area. This corresponds to the net application factor of 1.0. in real situation this factor varies from 0.73 to 1.14 (Allen, 1998). In Gaza Strip, a wide variety of crop pattern exists with no dominant crop. Thus, averaging this factor has minimal effect on the overall results.

 

        Other uncertainties could be related to meteorological data methods of measurements, recording, etc. but this is outside the scope of this term project.

 

 


9      FUTURE WORK

 

This work could be part of a regional water resources management project. So, estimates for other demand sectors; domestic, industrial, and other users could be done. This also could be linked with water resources availability in the region and water rights for different demand sectors.

 

The same type of study could be applied at the micro-scale for each crop type to give the exact crop water requirements. 

 


9      REFERENCES

 

Richard G. A.: Irrigation Engineering Principles, BIE 6010 Course Lecture Notes. Utah State University, 1998.

 

Richard G. A., Luis S. P., Dirk R., Martin S.: FAO - Food and Agriculture Organization of the United Nations - Irrigation and drainage paper 56. Crop evapotranspiration - Guidelines for computing crop water requirements - Rome, 1998.

 

Doorenbos J., Kassam A.H., and Bentvelson C.L.M: FAO - Food and Agriculture Organization of the United Nations - Irrigation and drainage paper 33. Yield response to water. Rome, 1979.