## 4 Results and Discussion
### Influence Factors
#### 4.2.2 Human Activities
This result can be explained by the water consumptions for paddy rice soaking before transplanting and for growth in the stage after transplanting being approximately 129 and 105 mm, respectively. The precipitation during these phases in 2010 was 48.5 mm, which is not enough to meet the water consumption for rice growth. Meanwhile, in 1998, the precipitation during the aforementioned phases reached 135.4 mm, which was enough to meet the consumption for rice growth. In dry years, the water demand of paddy tends to increase with the decrease in paddy areas. Similarly, this condition is due the precipitation in 2001, which was only 44.3 mm during the phase after rice transplanting. Given that this region has been suffering from serious drought since 1951, the precipitation was only 21.32 and 63.59 mm from the late spring to the early summer of 2011 [37], thus causing the water demand for paddy to amount to 2.4 \(\times\) 10\({}^{8}\) m\({}^{3}\). In April-May period, ponds have the greatest water consumption among all the land use types; thus, the water demands of aquaculture increase significantly with the increment of pond proportions in the wet and dry years. As shown in Figure 6b, the water demands of drought crops are close in the June-October period of the wet years. However, the water demand of paddy during the same period increases slightly with the decrease in the paddy area proportions. This increase is mainly caused by the minimal precipitation during the greenup stage. In the greenup stage in 1998, the water consumption for rice growth was 63.7 mm, but the precipitation was only 18.55 mm; hence, the water demand for the paddy was \(2.64\times 10^{8}\) m\({}^{3}\). It is the same on the reason of the greater water demand for paddy in 2010. In 1998, the water demand for aquaculture reached \(3.9\times 10^{8}\) m\({}^{3}\) because the minimal precipitation of 24.54 mm in September could not meet the required daily water consumptions for evaporation and infiltration. In dry years, the dryland area increased by 20.51 km\({}^{2}\) from 1990 to 2010, but the water demand for drought crops displayed a tendency to decrease. This finding mainly results from the lack of precipitation during the boll period in 1992. Meanwhile, in the dry years, the water demand of paddy tends to decrease with the decrease in paddy areas, and the water demand for aquaculture increases with the increment of pond proportions. In recent years, farmers have substituted cotton with corn because of the reduced economic benefits of the former. In addition, some measures, including paddy rotation to dry land and conduit hardening, have been performed to control the spread of snails [44,45]. Thus, the water demands for farmland have decreased. Further analysis of transfer matrix of land use showed that 80.86 percent of pond was converted from farmland and 18.67 percent of pond was converted from lake. If precipitation was enough to meet the consumption of agriculture at different stages, more water in Honghu Lake was diverted to farmland and pond through canals. Thus, water level would decrease and lake area shrinks. The pond area was 884.30 km\({}^{2}\) in 1990 and increased sharply to 1254.06 km\({}^{2}\) in 2010. Correspondingly, the water demand of aquaculture in 2011 was 2.29 times that in 1992 for dry years and in 2010 and 1.55 times that in 1989 for wet years. Evidently, the increase in pond area heightens the water demand of agriculture, which leads to quick reduction of the water-surface area.