Water debt for the annual production of nine major crops globally at 5 arcmin resolution for the year 2000

Definition of the Water Debt (WD) Repayment Time: The WD is calculated, at a 5' × 5' spatial resolution for the production of year 2000, as the ratio of the cell annual water footprint, WFs, cr, l, which is specific for each source s, crop cr, and location l, and the average renewable volume of water annually available in the cell. The annual renewability rate is calculated as a long-term average along the period 1987–2013. In this way, we obtain an average measure of the water availability in the cell, which smooths out potential extremely dry or extremely wet years. The water footprint (WF) estimates are computed for year 2000 because this is the most referenced year in agricultural datasets available in the literature. The WFs, cr, l estimate is obtained as the product of the local crop water footprint (CWF) by source, (CWFs, cr, l, expressed in cubic meters per ton of crop), and the annual crop production (Pr), (Pcr, l, expressed in tons of crop). The total WD of the sources, arising from all the crops cultivated in the grid cell, WDs, l, equals the sum of debts generated by each crop. Owing to the simultaneous replenishment of soil water, surface water, and groundwater by precipitation, the crop WD across the threes sources, WDcr, l, is given by the maximum WDs, l, cr value across sources s.

水债务（Water Debt, WD）偿还时间定义如下：本研究以2000年度生产数据为基础，采用5′×5′的空间分辨率开展计算，WD为某网格单元内，针对每种水源s、作物cr及区域l的单元年水足迹（WFs, cr, l）与该单元年平均可更新水资源量的比值。年更新率以1987–2013年期间的长期平均值计算，以此得到该网格单元水资源可获得性的平均水平，从而平滑极端干旱或极端湿润年份带来的波动。

选取2000年作为水足迹估算基准年的原因在于，该年份是现有农业数据集文献中引用最为广泛的年份。WFs, cr, l的估算值由按水源分类的单位面积作物水足迹（CWFs, cr, l，单位为立方米每吨作物）与年度作物产量（Pcr, l，单位为吨作物）的乘积得到。

网格单元内所有种植作物对应的各水源总WD（WDs, l）等于各作物产生的债务之和。由于降水可同时补给土壤水、地表水与地下水，因此针对单一作物cr及区域l的作物WD（WDcr, l）取各水源s对应的WDs, l, cr中的最大值。