Measuring the volume of irrigation water and water productivity of onion farms in three regions of Fars province

Onion, similar to most vegetables, is sensitive to inadequate irrigation. Onion has a shallow root and should be irrigated frequently with a small amount. Uniform and constant soil moisture during the growth period is very useful for onion growth. Excessive irrigation, in addition to reducing crop growth, increases costs, wastes water, reduces water productivity, and reduces product storability. Onion is one of the important agricultural products in Fars province. So far, little information has been published on the irrigation status and water productivity of onion fields. Knowing the amount of water required for irrigation and the water productivity of onion farms can help national and provincial planners and managers to increase production and save water more effectively. Therefore, the purpose of this research is to investigate the condition of onion farms in Fars province in terms of the amount of irrigation water and water productivity and the factors affecting them in agricultural conditions.

Yield, irrigation water volume and water productivity were measured and studied in 30 farms in three major onion production regions Shiraz, Sepidan and Kazerun in Fars province. T-test was used for statistical comparison of the obtained results. Three studied areas and three types of surface, sprinkler and drip irrigation systems were considered as experimental treatments. The daily meteorological information needed to estimate the water requirement was obtained from the synoptic weather stations of the regions using the Penman-Monteith method. Three scenarios were considered to estimate the water requirement of onion. 1- Water demand from the Penman-Monteith method, using the meteorological data of the year of conducting the research (2020) 2- Water demand from the Penman-Monteith method, using the meteorological data of 10 years leading up to the time of the research, 3- Water demand from the National Water Document. The average efficiency of surface, rain and drip irrigation systems was considered to be 60, 75 and 90%, respectively, and the gross water requirement was calculated. The farm water flow rates were measured with suitable instruments. The amount of irrigation water per hectare of the field was obtained by multiplying the amount of field discharge by the total duration of irrigation during the growing season. Finally, by summarizing the results obtained from the difference in yield values, irrigation water and water productivity, suggestions were made to improve onion production and irrigation.

The average volume of onion irrigation water in the three regions Shiraz, Sepidan and Kazerun was 15848, 14888 and 10507 m3/ha, respectively. The average total volume of irrigation water was 13880 m3/ha. The difference in irrigation water volume between Shiraz and Sepidan was not statistically significant at 5% level. However, the difference between the volume of irrigation water in Kazerun region and the other two regions was more than 4000 m3/ha, which became significant at the level of 1%. By applying 118, 327 and 195 mm of effective rainfall for the three investigated regions, the average amount of applied water in the fields of the three regions was 17028, 18158 and 12457 m3/ha respectively. The average volume of applied water in the three investigated areas was 16054 m3/ha. In terms of crop yield, the minimum, maximum, and average yield of the product in the three studied areas were about 34, 86, and 58 tons per hectare, respectively. The difference between Shiraz and Sepidan was less than one ton per hectare, which was not significant. However, the yield of Kazerun region was about 16 t/ha less than the yield of Sepidan and Shiraz regions, and these differences were significant at the level of 1%. The productivity of irrigation water in selected onion fields varied from 2.12 to 8.51 kg/m3 and on average was 28.4 kg/m3. The highest and the lowest irrigation water productivity were in Kazerun and Shiraz, respectively. The highest and lowest total water productivity was obtained in Shiraz and Sepidan regions. The difference in irrigation water productivity in the studied areas was not significant at the level of 5%. In Shiraz region, the amount of irrigation water given was 1003 and 723 m3/ha, respectively, less than the annual and long-term gross water requirements, and these differences were not significant at the 5% level. In Sepidan region, these differences were 1460 and 150 m3/ha, respectively, which was not statistically significant at the 5% level. In Kazerun region, the volume of irrigation water was 8427 and 7309 m3/ha, respectively, less than the gross water requirement, and these differences were significant at the 1% level. This means that the onion fields in this area did not receive the required water. In general, in the selected onion farms, the volume of irrigation water was less than the annual and long-term gross water requirement by 3419 and 2511 cubic meters per hectare, respectively, and these differences were significant at the 1% level.Comparison of irrigation water volume with gross water requirement showed that drip irrigation systems were most in line with gross water requirement and on average 5% more irrigation was done. However, in the fields under sprinkler and surface irrigation, 17 and 45% of water was given less than the gross water requirement, respectively. Therefore, the yield in onion fields under surface irrigation was about 28% lower than the yield in fields under drip irrigation system. The difference in irrigation water productivity between drip and surface irrigation systems was not significant.