Songjun Han

Positive or negative correlation between actual and potential evaporation? Evaluating using a nonlinear complementary relationship model

Understanding whether a positive or negative correlation between actual (E) and potential (Ep) evaporation is of great importance in detecting changes of E from Ep. In this study, such correlation was evaluated via the linear regression slope (k) between E and Ep derived from a nonlinear complementary relationship model. k varies with the relative degree of variability in the radiation term (Erad) and aerodynamic term (Eaero) of Ep, and is further affected by water availability. The sign of k is always positive under conditions with high Erad variability but at the same time with low Eaero variability (commonly true on an hourly basis). Under conditions with high Eaero variability but at the same time with low Erad variability, the sign of k tends to change from negative to positive with more readily water availability. On a daily or annual basis, the sign of k would be related with water availability. Under water-limited conditions, the Eaero variability is more significant than the Erad variability, and negative k is expected. Under energy-limited conditions, the Erad variability is compared to or even much obvious than the Eaero variability, and positive k is expected. This implies a negative correlation between E and Ep under water-limited conditions but a positive correlation under energy-limited conditions on the daily or annual basis. The above analysis is confirmed in a grassland site in Northeast China at daily and half-hourly time scales. The correlation between annual E and Ep over China was evaluated using data of 690 meteorological stations from 1956 to 2005. The k values change from negative in arid regions to positive in humid regions. According to k and the decreasing trends in annual Ep, it is deduced that E increased for most stations in arid regions but decreased for most stations in humid regions.

Cooling effect of agricultural irrigation over Xinjiang, Northwest China from 1959 to 2006

The influences of agricultural irrigation on trends in surface air temperature from 1959 to 2006 over Xinjiang, Northwest China are evaluated using data from 90 meteorological stations. The 90 stations are located in landscapes with markedly different cultivated land uses. The increasing trends in daily average temperature.Ta/, maximum temperature.Tmax/, and minimum temperature.Tmin/ for May‐September (the main growing season) are negatively correlated with cultivated land proportions within 4 km of the meteorological stations, as indicated by year 2000 land use data. The correlations between the trends in Tmax and cultivated land proportions are the most significant. The trends in Ta, Tmax, and Tmin for May‐September are expected to decrease by 0.018, 0.014, and 0.016 C per decade, respectively, along with a 10% increase in cultivated land proportion. As irrigated cultivated land occupies over 90% of total cultivated land, the dependence of temperature trends on cultivated area is attributed to irrigation. The cooling effects on stations with cultivated land proportion larger than 50% are compared to temperature trends in a reference group with cultivated land proportion smaller than 10%. The irrigation expansion from 1959 to 2006 over Xinjiang is found to be associated with cooling of May‐September Ta, Tmax, and Tmin by around 0.15 C to 0.10 C/decade in the station group with extensive irrigation. Short periods of rapid irrigation expansion co-occurred with the significant cooling of the May‐September temperature.

Similarities and differences of two evapotranspiration models with routinely measured meteorological variables: application to a cropland and grassland in northeast China

Local actual evapotranspiration can be estimated with routinely measured meteorological variables using the Penman–Monteith model with surface resistance parameterized via the Katerji and Perrier approach (Agronomie 3(6):513–521, 1983; PM–KP model), or the nonlinear complementary relationship (CR) model proposed by Han et al. (Hydrol Process 26:3973–3981, 2012). A comparative study was carried out to evaluate the consistencies and differences of two models, as well as the performances of them for a cropland and grassland in northeast China. The departure of the actual evapotranspiration from the potential evaporation is described as a function of the ratio of the surface resistance to the aerodynamic resistance in the Penman–Monteith model, but the ratio of the aerodynamic term to the radiation term in the CR models. The two ratios are connected using a semi-empirical linear function by the Katerji and Perrier approach. The nonlinear CR model can be regarded as replacing the linear function by a power function after mathematical processing. On the other hand, the PM–KP model can be also considered as a CR-type model. On the daily basis at a maize cropland and degraded grassland in semiarid Northeast China, the nonlinear CR model with locally calibrated parameters performed better with data occupying all the growth stages, but the performances of the two models are similar during the early-, mid-, and late-season stages, respectively. On the half-hourly basis, the PM–KP model and the nonlinear CR model both performed well. It is deduced that on the daily basis the nonlinear CR model is more suitable for the cropland and grassland, but further comparisons are needed on the hourly basis.

Water Requirement with Irrigation Expansion in Jingtai Irrigation District, Northwest China: The Need to Consider Irrigation-Induced Local Changes in Evapotranspiration Demand

AbstractBecause water diverted from the Yellow River has been used for irrigation in 1972, obvious changes in evapotranspiration demand (denoted by reference evapotranspiration) have been observed in the Jingtai Irrigation District, Northwest China. Increasing irrigation-water consumption corresponding to irrigation expansion led to less local evapotranspiration demand after 1972. The crop and irrigation-water requirements per unit area were influenced by the changes in evapotranspiration demand. A simple prediction approach based on the complementary relationship of evapotranspiration was used to estimate the irrigation-induced changes in evapotranspiration demand and their influences on irrigation-water requirement. Future variations in net irrigation requirement per unit area under different irrigation scenarios at 50% and 75% precipitation levels were evaluated. Irrigation requirements per unit area would decrease under irrigation-expansion scenarios but increase under irrigation-reduction scenarios c...

Surface wind observations affected by agricultural development over Northwest China

Meteorological stations in Northwest China are surrounded by large proportions of cultivated land. The relations between the change of surface wind speed and the cultivated land fractions (CF) within a 4 km radius at 135 meteorological stations over arid Northwest China are investigated. Stations with larger CF experienced larger declines in surface wind speed from 1960 to 2007. Compared with the wind speed variation in the Tibetan Plateau where agricultural development is negligible, stations with low CF show similar variation, whereas the wind speed at stations with large CF illustrates a sharp decrease in the 1970s–1980s, during which irrigated agriculture developed rapidly. The observed wind speed at the station surrounded by irrigated fields in the Jingtai Irrigation District, shows a rapid wind speed decrease during the same period when the irrigated area expanded. By contrast, rapid wind decrease is not observed at a nearby station with minimal influence of agricultural development.

Observed near-surface atmospheric moisture content changes affected by irrigation development in Xinjiang, Northwest China

The effects of irrigation development on observed near-surface atmospheric moisture changes remain unclear in arid Xinjiang. In this study, cultivated land fractions (CFs) within a 4-km radius of 90 meteorological stations over Xinjiang, which are inferred from the 2000 land use map, are used as a quantitative indicator of irrigation intensity. Trends of observed water vapor pressure and relative humidity during the growing season (May to September) from 1959 to 2006 are significantly positively correlated with CFs of the meteorological stations. Stations with larger CFs experience a more rapid increase in near-surface atmospheric moisture than stations with small CFs. Results indicate that growing season near-surface atmospheric moisture wetting is enhanced by irrigation development for stations with high levels of cultivated land uses. The land use around stations should be considered when analyzing the observed near-surface atmospheric moisture changes in Xinjiang.

Impacts of urbanization and agricultural development on observed changes in surface air temperature over mainland China from 1961 to 2006

A large proportion of meteorological stations in mainland China are located in or near either urban or agricultural lands that were established throughout the period of rapid urbanization and agricultural development (1961–2006). The extent of the impacts of urbanization and agricultural development on observed air temperature changes across different climate regions remains elusive. This study evaluates the surface air temperature trends observed by 598 meteorological stations in relation to the urbanization and agricultural development over the arid northwest, semi-arid intermediate, and humid southeast regions of mainland China based on linear regressions of temperature trends on the fractions of urban and cultivated land within a 3-km radius of the stations. In all three regions, the stations surrounded by large urban land tend to experience rapid warming, especially at minimum temperature. This dependence is particularly significant in the southeast region, which experiences the most intense urbanization. In the northwest and intermediate regions, stations surrounded by large cultivated land encounter less warming during the main growing season, especially at the maximum temperature changes. These findings suggest that the observed surface warming has been affected by urbanization and agricultural development represented by urban and cultivated land fractions around stations in with land cover changes in their proximity and should thus be considered when analyzing regional temperature changes in mainland China.