ASSESSMENT OF BLUE, GREEN AND GREY WATER FOOTPRINT AT RIVER BASIN LEVEL: A CASE STUDY OF THE KOSHI RIVER BASIN, NEPAL

Abstract

Nepal is an agrarian country and almost one-third of Gross Domestic Product (GDP) is dependent on agricultural sector. Koshi river basin is the largest basin in the country and serves large share on agricultural production. Like another country, Nepalese agriculture holds largest water use in agriculture. In this context, it is necessary to reduce water use pressure. In this study monsoon rainfall variability in the Koshi basin have been analyzed in different scale seasonal, monthly, decadal and daily for the period of 1980-2015, and water footprint of different crop (rice, maize, wheat, millet, sugarcane, potato and barley) have been estimated for the year 2005 -2014 to get the average water footprint of crop production during study period. A sample calculation of water footprint has been shown on the appendix. The current study of water footprint estimation is based on CROPWAT model developed by Food and Agriculture Organization (FAO 2010b) that requires climatic data, soil properties, and crop characteristics as input. This method calculates evapotranspiration from a field, which is the water consumption in field level. Medium soil and 70 % irrigation efficiency is considered in irrigation schedule option, and ET of the crop is calculated considering daily moisture balance for both rainfed and irrigated scenario in the model. It doesn’t include conveyance losses during irrigation process. So, water footprint production of any crop (Mm3/year) doesn’t represent the irrigation diversion requirement but represents the crop water use at the field for producing that crop during the study period. Both the green and blue water footprint estimation is based on ET (the output of CROPWAT model) and yield (derived from statistical data). Blue and green water footprint are calculated for different districts (16 districts within KRB) / for KRB in different years (10 years from 2005 to 2014) and crops (considered 7 local crops). The water footprint of crops production at any district or basin represents the averaged of WF production of seven crops in the respective district or basin. Then trend analysis of each crop is also carried out for basin level. This study gives a picture of green and blue water use in crop production in the field and gives the idea of reducing the water footprint of crop production by selecting suitable crop at a suitable place. Water footprint of sugarcane has been significantly decreased after the introduction of higher yielding or diseases resistant variety of in the year 2010. In the year 2009, most of the crop must tolerate water stress. In this year rainfall is less than other years and recognized as a drought year. To meet crop water requirement irrigation water has been supplied more in that year. So, the blue water use is significantly increased. Crop that has lower water footprint can be intensified in that location and the crop having higher water III footprint can be discontinued for production or measure for water saving technique needs to be implemented to reduce evapotranspiration. The water footprint of agriculture crop production can be reduced by increasing the yield of the crops. Some measures like use of an improved variety of seed, fertilizer, mechanized farming and soil moisture conservation technology can be used to increase yield. The crop harvested area in this study includes both rainfed as well as irrigated land. Agricultural land occupies 22% of the study area, of which 94% contributes rainfed and 6% contributes irrigated area. The study shows 98% of total water use in crop production is due to green water use (received from rainfall) and remaining 2 % is due to blue water use received from irrigation (surface and ground water as source). Potato has 22% blue water proportion and contributes 85% share on total blue water use in the basin. Maize and rice together hold 77% share of total water use in crops production. The average annual water footprint of crop production in KRB is 1248m3/ton with the variation of 9% during the period 2005-2014 Sunsari, Dhankuta districts have lower water footprint of crop production. The coefficient of variation of water footprint of millet crop production is lower as compared to those of other crops considered for study whereas sugarcane has a higher variation of water footprint for its production.