**2.3. Determination of water footprint differences of cultivars of selected crops**

Water footprint (m3 t ha−1) was determined as the ratio of actual crop water use (m3 ha−1) to the total yield or total biomass yield (t ha−1) [10]. Total water footprint was the sum of blue, green and grey water footprint. Blue and green water footprint (m<sup>3</sup> t ha−1) was a ratio of blue and green crop water use (mm), to the total yield or total biomass yield (t ha−1), respectively [18]. Grey water footprint (m3 t ha−1) was determined as a ratio of total volume of water (m3 ) required diluting nitrogen that reached the ground water, per ton of produce [19]. Grey water footprint was estimated by multiplying the leaching fraction by the nitrogen application (kg ha−1) and dividing the difference between the permissible limit and the natural concentration of nitrogen in the receiving water body. The study assumed a natural water nitrate concentration of 5.6 mg l −1 and the permissible limit of 11.3 mg l −1 [20]. Leaching fraction was assumed at 10% [18, 21]. This study compared the water footprint based on actual crop yield and crop water use, in order to remove the disparity of over-estimation, once hypothetical crop and crop water requirements are used [22, 23].

#### **2.4. Social-economic analysis of the selected crop cultivar**

An economic assessment of Taewa against modern potato varieties in relation to irrigation investments was done using the net present value (NPV) method. Net present value is an investment analysis also referred as a total of present value of a single project cashflow of the same unit [24]. In order to get NPV, fixed and annual operating costs and expected returns were estimated based on a 5-ha small scale irrigation using a Trail Travel Irrigator to obtain the economic implications of the system on crop production. The data in the study on marketable fresh tuber or marketable fruit yield were used to analyse the economics of Taewa and water footprint. Crop water use and total yield from the three crops were pooled, in order to determine their comparative water footprint differences.
