PERFORMANCE ASSESSMENT OF GEDIZ BASIN
USING SECONDARY DATA
Göksel GEÇGEL*, Atila GİRGİN*, Müslüm BEYAZGÜL* ,
Charlotte de FRAITURE** and Hammond Murray-Rust**
*Tarımsal Hidroloji Araştırma ve Eğitim Merkezi, 35660 Menemen, İZMİR, TURKEY
Phone: +90 232 831 10 52 – 831 05 12, fax: +90 232 831 10 51 – e-mail: email@example.com
**International Water Management Institute, P.O. Box 2075 Colombo Sri Lanka, Phone:+94-1-867404
fax: +94-1-866854, e-mail: H.Murrayrust@cgiar.org
This study provides an overview of the developments in irrigated agriculture in the Gediz basin over the last 15 years. The analysis is based on data derived from DSI and the Met Dept. These data and underlying assumptions were checked using information collected by IWMI-GDRS during fieldwork and surveys conducted during the 1998 irrigation season. Further, a LANDSAT image covering the basin was used to compare irrigated area reported by DSI/WUA and actual irrigated areas. It was concluded that the DSI data were accurate enough for the analysis made in this study. Concluding remark; despite the reduction in surface water farmers were able to increase land and water productivity.
This study focuses on the large scale irrigation systems. As part of the GDRS-IWMI program data on climate, cropping pattern, irrigation water use, yields and prices were collected from secondary sources. The data were analyzed making use of the irrigation performance indicators developed by IWMI (Perry 1996, Molden et al. 1998). The accuracy of the data was checked by comparing them with field observations during the 1998 irrigation season
Main objectives of the study are given below :
¨ To obtain a general overview of developments in irrigation practices and agricultural performance in the Gediz Basin. This information provides the setting for in-depth studies on tertiary canal level concerning irrigation practices and groundwater use by individual farmers.
¨ To evaluate the accuracy and usefulness of secondary data in irrigation performance assessment studies and trend analysis.
MATERIALS AND METHODS
In the Gediz Basin there are 7 large scale irrigation schemes and a few hundred small scale irrigation systems. This study focuses on the large scale irrigation systems. As part of the GDRS-IWMI program data on climate, cropping pattern, irrigation water use, yields and prices were collected from secondary sources. The data set covers a time span of 15 years (1984-1998). Until 1995 the main data source is DSI, the government agency responsible for the operation and maintenance of the large irrigation schemes. In 1995 the management was transferred to the newly formed Water Users Associations. Several WUA offices were visited to collect data for the period 1995-98.
Details of the systems in the study are given below:
Table 1. Main irrigation scheme and associations in Gediz basin
Note: the smallest of the 7 large scale systems (Akpinar, 1000 ha) was excluded from the analysis due an incomplete and inconsistent data set.
The data were analyzed making use of the irrigation performance indicators developed by IWMI (Perry 1996, Molden et al. 1998). The accuracy of the data was checked by comparing them with field observations during the 1998 irrigation season. Further, a LANSAT image from the Gediz Basin was used to compare irrigated areas and cropping patterns with the reported data given by DSI and Water Users Associations.
Surface water supply
After 1989 the amount of surface water supplied from the reservoirs in the basin dropped substantially due to a severe and prolonged drought. During the period 1985-88 the total amount of surface water supplied to the six schemes fluctuated between 700 and 800 million cubic meter, while in the period 1989-93 the total surface supply varied between 100 and 200 million m3. During the driest year (1992) the overall water supply was as low as 15% of the supply before the drought. Over the last four years the situation is improving again: in 1995 the supply was at 45% of the pre-drought level, while in 1997 the water supply was 50%.
The surface water supply per scheme varies according to the location of the scheme in the basin. The graph below indicates the water supplied from the reservoirs expressed as water layer on the command areas, per zone. The basin is divided in three zones: the upper valley, main valley and the delta.
The graph clearly shows the enormous drop in water supply which was most pronounced in the main valley, covering 66% of the irrigated area. However care should be taken while interpreting the water supply data as reported by the DSI. The DSI only reports the releases from the reservoirs during the irrigation season (July-September) and doesn’t take into account the reuse of drainage water or water originating within the catchment below the reservoir. For example, the Manisa scheme (nearly half of the area in the main valley) receives drainage water which is not visible from the DSI data and consequently, the water supply in main valley will be higher than appears from the graph. Unfortunately, data or estimates of drainage water reuse and are not available. Despite this uncertainty in data in is clear that there was a considerable reduction in surface water supply. In the next paragraphs the responses of water managers and farmers to this reduction will be examined.
Responses to drought
Possible responses to drought include:
a) reduce the irrigated area
b) change in cropping pattern
c) reduce field application and stress crops
d) pump ground water
During the period before the drought around 80% of the total command area was irrigated with water releases from the reservoirs. After 1989 this percentage dropped to 50% while it went up again to around 60% in 1997 and 1998. The graph below shows the percentage of the command area irrigated by surface water, per zone in the basin. In the delta the irrigation intensity is highest (80% over the last three years), and in the middle part of the basin lowest (55%).
From the graph it is clear that the irrigated area was reduced during the drought. However, it should be noted that that the DSI only keeps records of the area irrigated by surface water issued from the reservoirs. Area exclusively irrigated by groundwater does not appear in the published records and no information is available about groundwater use. It might be that in reality the total area irrigated did not decrease at all because the area exclusively irrigated by groundwater - invisible in the records - replaced the area under surface irrigation. This subject is further explored in paragraph 2.3
Changes in cropping pattern
The main crops grown in the basin are grapes and cotton. Cotton is mainly grown in the delta while grapes is the predominant crop in the upper valley. As can be seen from the graphs below, the area cultivated with cotton in the upper and main valley is gradually decreasing while the area with grape is increasing. The reduction in area with cotton is most outspoken in the upper valley where practically no more cotton is grown after 1992. In the delta cotton remains the predominant crop, cultivated on 70% of the irrigated area because soils are more suitable for cotton than for grapes. Overall in the basin, the percentage of the irrigated area with cotton remains more or less on the same level, around 50%. In absolute terms the area decreases from 52,350 ha (49% of command area) in 1984 to 35,475 ha (32% of command area) in 1996.
The coverage of grapes, the second important crop in area, is steadily rising from 20% to nearly 35% of the irrigated area. In absolute terms the area roughly doubles from 11,760 ha in 1984 (11% of the command area) to 23,600 ha in 1998 (22% of irrigated area).
One could argue that the change from cotton to grapes was induced by water shortage since grapes are less susceptible to yield reduction due to crop stress than cotton. However, crop choice is influenced by many factors and water availability is only one of them. Grapes is a perennial crop that requires high initial investment, and it is unlikely that farmers replaced cotton for grapes just because of a (temporary) drought. Other factors such as prices and markets probably played a more important role in crop choice. Both cotton and grapes prices and yields show an increasing trend over the last decade, but the increase for grapes is more pronounced than for cotton.
Reduce water layer and stress crops
If farmers chose to reduce the water layer applied and stress the crops, one would expect a reduction in yield. However, the general trend in cotton yield is a rising line from 2.3 ton/ha in 1984 to 3.0 ton in 1996. Grape yields improved from 4.0 ton/ha to about 5.5 ton/ha in 1996. Therefore, from the DSI data there is no evidence that farmers reduced the water layer and stressed crops.
There are considerable differences in cotton yields between the systems. In the delta (Menemen) where cotton is the main crop, yields are high compared to the yields in the upper valley where cotton is hardly grown. In lower parts of the basin (Menemen and Manisa) where grapes are hardly grown, the yields are low in comparison to those in the upper part of the valley where grapes are the main crop. Those differences might be caused by variations in soil or micro-climate and explain why in the lower part of the valley grapes are hardly grown despite the better prices for grapes.
Groundwater in conjunction with surface water
After 1989, the first year of the drought, farmers started investing in tubewells to supplement surface water supply. Unfortunately, there are no data available about the number of wells and the amount of groundwater pumped, since the irrigation agency only kept records of water releases from the reservoir. In this study the amount of groundwater used in conjunction with surface water is estimated, assuming that the irrigation efficiency before and after the drought remains on the same level. For the period after 1989-91 the groundwater supply is estimated as the difference between gross irrigation requirements and surface water supply. According to this estimation method, in the period 1990-94 about 60% of the water supply came from groundwater. After 1994 this amount decreases to 30% due to increased surface water supply.
Area irrigated exclusively by groundwater
From field observations:
Farmers continue using groundwater in conjunction with surface water, but also irrigate considerable area exclusively with tubewells. Reasons:
· convenient: always water available at desired time and amounts
· cheaper than surface water: WUA increased the surface water fees considerably over the last few years to cover their operation and maintenance costs. According to some farmers the costs to pump are less than the surface water fees by the WUA. However, most farmers don’t account for investment and amortization costs
Trends in land and water productivity
The total production
The total production of irrigated agriculture in the main irrigation schemes in the Gediz basin, expressed in 1995 constant Turkish Lira, remained more or less on the same level over the last decade despite the enormous reduction in surface water supply. The first year of the drought (1989) the production dropped due to reduced area, but thereafter it increased steadily. Firstly because the irrigated area increased after farmers started investing in tubewells and secondly because yields and prices improved.
Figure 1.The surface water supply
Figure 2. The percentage of the command area irrigated
Figure 3. The percentage of irrigated area with cotton
Figure 4. The percentage of irrigated area with grapes
Figure 5. The total production in Gediz basin
Figure 6. The production per unit command area in Gediz basin
Figure 7. The production per unit irrigated area
Figure 8. The production per unit of water consumed by evapotranspiration
The production per unit of irrigated area is steadily increasing due to improved yields and prices of the two main crops (grapes and cotton). This increase is most pronounced in the upper part of the basin where cotton gradually was replaced by grapes. As mentioned before compared to cotton, grape prices and yields improved more. In the delta the production expressed in monetary terms is lower due to bigger cotton area.
The production per unit command area increases slightly. At one hand the production per area irrigated increases. At the other hand percentage of the command area that is irrigated (with surface water) is decreasing. These opposite factors result in a slightly increasing output of the systems. The differences between upper and lower part of the valley are less pronounced than in case of the production per unit of irrigated land. In the
upper part farmers switched increasingly to a higher yielding crop such as grapes, but at the same time reduced the area irrigated. In the lower valley farmers continue to grow cotton with lower output per hectare, but they only slightly decreased the area irrigated with surface water.
The production per unit of water consumed by evapotranspiration roughly follows the same pattern as the production per unit irrigated area. Changes in yields and prices are the main determinants of water productivity trends, because the ETcrop remains more or less on the same level (variations in climate over the years are negligible). The rise in water productivity is of the same level of magnitude as in land productivity.
Despite the reduction in surface water farmers were able to increase land and water productivity.
This study was conducted as part of a collaborative research program between the General Directorate of Rural Services (GDRS) in Turkey and the International Water Management Institute (IWMI) and is funded by the Government of Turkey as part of the World Bank assisted Turkish Agricultural Research Plan (TARP).
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