CHAPTER 4 - RAINFALL AND EVAPOTRANSPIRATION
 Soil moisture control on evapotranspiration is poorly understood in ecosystems Spatial relation between percentage of total annual rainfall. Relationship Between Evapotranspiration and Precipitation Pulses in a Semiarid Rangeland Estimated by Moisture Flux Towers and MODIS. Relationship between evapotranspiration and precipitation pulses in a semiarid rangeland estimated by moisture flux towers and MODIS vegetation indices.
Another reason is that EC towers cannot capture the large eddies with low frequency associated with stationary secondary circulations that generate over tall canopies and heterogeneous landscapes [ 35 — 39 ]. Regional Climate Data Climate data, including daily precipitation, temperature, air humidity, sunshine duration, and wind speed, were obtained from the National Climate Center, China. At the time of study, there were meteorological stations across the Loess Plateau.
When analyzing the domain-average trend of these climate variables, we averaged them for the stations. Vegetation Index Vegetation indices are radiometric measures of photosynthetically active radiation absorbed by chlorophyll in the green leaves of vegetation canopies and are therefore good surrogate measures of the physiologically functioning surface greenness level of a region.
Advances in Meteorology
During data preprocessing, we spliced the images, transformed the projection, and extracted the study area the Loess Plateau. To date, an efficient way to overcome this problem remains elusive. However, in situ ET data can provide a reference for the simulated ET [ 40 ]. It can be seen that the simulated ET was highly consistent with the measured ET at the four validation sites. As shown in Table 2the fitting coefficient was close to 1, the average coefficient of determination was about 0.
Correlation statistics between predicted ET and observations at four sites in the Loess Plateau. Results and Discussion 3. It can be seen that ET decreased from to over the Loess Plateau, at a rate of 0.
The decreasing trend was weak before and then accelerated after Figure 3 b shows the time series of the ratio of ET to precipitation, which also shows an obvious decreasing trend.
The decreasing trend implies that ET decreased more sharply than precipitation during the study period; water recycling from the land surface to the atmosphere was becoming weaker, as was the regional water cycle.
Besides, the standard deviation for the s was much larger than that of the s and s, implying that ET fluctuated abnormally strongly in the s, which could have caused disturbance of the water cycle leading to drought [ 42 ]. Interdecadal characteristics of ET over the Loess Plateau.
The ET trend in each season and their contribution to the overall ET trend were also investigated Figure 4.
Evapotranspiration And Precipitation
For each year, the seasonal ET was calculated by summing the monthly ET within the separate seasons. It can be seen that ET decreased in all seasons except autumn. Summer had the largest magnitude, accounting for around half of the annual ET. ET also decreased most rapidly during summer, at a rate of 0. In spring, ET fluctuated strongly, especially aroundand had a relatively small rate of decrease. ET had a small mean and rate of decrease in winter, and in autumn, ET increased slightly but nonsignificantly.
Overall, the summer trend dominated the declining ET trend over Loess Plateau. Trend of ET in each season over the Loess Plateau during — Local ET Trend over the Loess Plateau Although the domain-average ET decreased over the Loess Plateau during —, it was possible that local ET trends may have differed due to the complicated topography and distinct features of local climate.
Figure 5 shows the local ET trends over the Loess Plateau during the study period. This small increasing ET trend was dominated by the larger decreasing ET trend, meaning the domain-average ET showed an overall decreasing trend. The areas with an increasing ET trend were located in Tianshui District and the intersection of the Yellow River, Luo River, and Wei River, where there is a relatively high level of water availability.
Distribution of the climate tendency of ET over the Loess Plateau. Figure 6 further breaks down the ET trend into that of the semiarid and that of semihumid region of the Loess Plateau. It can be seen that ET was much larger in the semihumid region than in the semiarid region. Furthermore, ET decreased more rapidly in the semiarid region than in the semihumid region, with their rates of decrease being 0. A larger availability of water slowed the rate of decrease in the semihumid region.
Trend of ET in the semiarid and semihumid areas of the Loess Plateau. Attribution of the ET Trend To elucidate the factors causing the decreasing ET trend, we examined the temporal evolution and trends of annual precipitation, air humidity, temperature, sunshine duration, wind speed, and vegetation conditions.
Annual precipitation and air humidity showed an obvious decreasing trend during the study period Figures 7 a and 7 bmeaning that water availability was decreasing over the Loess Plateau. The trend in sunshine duration was not significant Figure 7 c.
Temperature and wind speed showed marked increasing trend, indicating that the evaporation potential was increasing Figures 7 d and 7 e. NDVI also increased Figure 7 fwhich may have contributed to an increase in transpiration.
These results indicate that the declining trend in annual precipitation and air humidity caused the decreasing trend in ET, which outweighed the effects of increasing temperature, wind speed, and NDVI. Trends of annual a precipitation, b air humidity, c sunshine duration, d air temperature, e wind speed, and f NDVI over the Loess Plateau. Increased potential ET [ 2425 ] and decreased actual ET imply that a complementary relationship between the two exists over the Loess Plateau.
Similar research on different regions of China [ 17 ] showed that potential and actual ET were both controlled by available energy and both showed decreasing trends in humid and semihumid regions, while in arid and semiarid regions, potential ET was controlled by available energy and showed a decreasing trend, and actual ET was controlled by available water and showed an increasing trend.
The Loess Plateau, however, shows an opposite ET trend compared to the arid and semiarid regions of China as a whole. The increasing ET trend in other arid and semiarid regions was attributed to increased precipitation, relative humidity, and cloud cover [ 43 ]. Our results showed that water availability, represented by precipitation and air humidity, was decreasing during the study period, which caused the decreasing trend of actual ET.
In the study area, potential evapotranspiration ET is the upper limit of evapotranspiration when there is no water limit and represents the evaporative demand.
Actual evapotranspiration is evapotranspiration mainly limited by the water supply of the area. Potential ET is higher than actual ET; it means that the ET demand is higher than the actual ET, indicating that precipitation cannot meet the ET demand and all precipitation except run-off transforms into ET, resulting in a better correlation between actual ET and precipitation.
Therefore, the different local ET trends may be related to local water availability. Figure 8 shows the relationship between the climate tendency rate of local ET and mean annual precipitation.
It can be seen that the climate tendency rate of local ET was closely related to local mean annual precipitation. The climate tendency rate of local ET changed from negative to positive when local mean annual precipitation increased. This implies that ET changed from being supply-limited to energy-limited due to increased water availability.
The climate tendency rate was negative in supply-limited areas where local mean annual precipitation was less than mm, because water supply was decreasing in these areas; it could be negative or positive in the transition zone where local mean annual precipitation was within the range — mm, and it was positive in energy-limited areas where local mean annual precipitation was greater than mm, because the ET potential was increasing and water availability was sufficient in these areas.
In the semiarid region, local mean annual precipitation was less than mm, so the local climate tendency was negative and local ET showed a decreasing trend. In most of the semihumid region, local mean annual precipitation was within the range — mm, so the local climate tendency was either negative or positive and local ET showed a decreasing or increasing trend in different areas.
For the intersection areas of the Yellow River, Luo River, and Wei River, local mean annual precipitation was greater than mm, the local climate tendency rate was positive, and the local ET showed an increasing trend. Relationship between the climate tendency rate of ET and local mean annual precipitation.
The strong solar radiation of the Loess Plateau means relatively high levels of available energy, and therefore the only limit for ET is the availability of water. An increase in water resources leads to changes in the climate tendency rate of ET from negative to positive, and this causes two opposing feedback mechanisms in the region. In areas with little precipitation less than mmthe decreasing ET contributes less moisture to the atmosphere and decreases precipitation locally, further intensifying aridity.
In areas with large quantities of precipitation greater than mmthe increasing ET contributes more moisture to the atmosphere and increases precipitation locally, further intensifying humidity.
These mechanisms are quite different from those in humid regions, where ET increases under low-precipitation conditions and decreases under high-precipitation conditions, due to available energy being reduced under high-precipitation conditions [ 44 ]. The domain-average ET decreased during the study period, at a rate of 0. In particular, the decreasing trend accelerated afterwith ET decreasing more from the s to s compared to the s to s.
Introduction When rain water 1 in Fig. When the rainfall stops, some of the water stagnating on the surface 3 evaporates to the atmosphere 5while the rest slowly infiltrates into the soil 6.
From all the water that infiltrates into the soil 2 and 6some percolates below the rootzone 7while the rest remains stored in the rootzone 8. The term effective rainfall is used to define this fraction of the total amount of rainwater useful for meeting the water need of the crops. Factors influencing effective rainfall Many factors influence the amount of the effective rainfall. There are factors which the farmer cannot influence e. Climate The climate determines the amount, intensity and distribution of rainfall which have direct influence on the effective rainfall see 4.
Soil texture In coarse textured soil, water infiltrates quickly but a large part of it percolates below the rootzone.
In fine textured soil, the water infiltrates slowly, but much more water is kept in the rootzone than in coarse textured soil. Soil structure The condition of the soil structure greatly influences the infiltration rate and therefore the effective rainfall. A favourable soil structure can be obtained by cultural practices e.
Depth of the rootzone Soil water stored in deep layers can be used by the plants only when roots penetrate to that depth. The depth of root penetration is primarily dependent on the type of crop, but also on the type of soil. The thicker the rootzone, the more water available to the plant. Effective rainfall and depth of the rootzone e. Topography On steep sloping areas, because of high runoff, the water has less time to infiltrate than in rather flat areas see Fig.