Guofu Yuan

Evaluation of a crop water stress index for detecting water stress in winter wheat in the North China Plain

Canopy temperature measured with infrared thermometers or other remote infrared sensors is an important tool for detecting crop water stress. The crop water stress index (CWSI) is the most often used index which is based on canopy temperature to detect crop water stress. This study evaluates the application of three different forms of CWSI for winter wheat water stress monitoring in the North China Plain (NCP): the Idso empirical model, the Jackson theoretical model, and the new Alves model, which replaces the radiometric surface temperature with a surface “wet bulb” temperature, thereby avoiding the evaluation of the surface resistance of the crop. The results show that the CWSI based on Jackson’s definition and Alves’ definition are better than the empirical CWSI for monitoring winter wheat water stress in NCP. Both definitions are useful tools to evaluate winter wheat water stress in NCP, but the CWSI based on Alves’ definition is more practical for monitoring winter wheat water stress in NCP, mainly due to not having to estimate the crop surface resistance, while the CWSI based on Jackson’s definition is more reasonable for quantifying winter wheat water stress in NCP.

EVALUATION OF MACROSCOPIC ROOT WATER UPTAKE MODELS USING LYSIMETER DATA

Three macroscopic root water uptake models (Molz-Remson, Feddes, and Selim-Iskandar) have been widely used in modeling water flow in soil-plant systems. In this article, we use evapotranspiration data and soil water content data obtained from lysimeter measurements and root distribution data obtained in a nearby wheat field to evaluate the accuracy of these root water uptake models in predicting the soil water content profiles in the winter wheat field. To account for the role of root distribution, we modified the Molz-Remson model and the Selim-Iskandar model with the Feddes reduction function, and the Feddes model with the root length density. These models, with and without modifications, were then individually incorporated into the soil water flow equations as a sink term. The flow equations were solved numerically with the measured evapotranspiration data as input, and the predicted soil water content profiles were compared with the measured profiles to evaluate the validity of the root water uptake models. The comparison showed that: (1) the average deviation of the Molz-Remson (linear version) model, the Feddes model, and the Selim-Iskandar model could reach as high as 25,70, and the non-linear version of the Molz-Remson model performed slightly better, with an average deviation of 13%; (2) modifications made to the Molz-Remson models and the Selim-Iskandar model did not achieve any improvement in the model predictions, but the modified Feddes model significantly improved the prediction accuracy, reducing the average deviation to 5.6%.

Total Nitrogen Concentrations in Surface Water of Typical Agro- and Forest Ecosystems in China, 2004-2009

We assessed the total nitrogen (N) concentrations of 28 still surface water (lake and pond), and 42 flowing surface water (river), monitoring sites under 29 typical terrestrial ecosystems of the Chinese Ecosystem Research Network (CERN) using monitoring data collected between 2004 and 2009. The results showed that the median total N concentrations of still surface water were significantly higher in the agro- (1.5 mg·L−1) and oasis agro- ecosystems (1.8 mg·L−1) than in the forest ecosystems (1.0 mg·L−1). This was also the case for flowing surface water, with total N concentrations of 2.4 mg·L−1, 1.8 mg·L−1 and 0.5 mg·L−1 for the agro-, oasis agro- and forest ecosystems, respectively. In addition, more than 50% of the samples in agro- and oasis agro- ecosystems were seriously polluted (>1.0 mg·L−1) by N. Spatial analysis showed that the total N concentrations in northern and northwestern regions were higher than those in the southern region for both still and flowing surface waters under agro- and oasis agro- ecosystems, with more than 50% of samples exceeding 1.0 mg·L−1 (the Class III limit of the Chinese National Quality Standards for Surface Waters) in surface water in the northern region. Nitrogen pollution in agro- ecosystems is mainly due to fertilizer applications, while the combination of fertilizer and irrigation exacerbates nitrogen pollution in oasis agro- ecosystems.

Evapotranspiration and its main controlling mechanism over the desert riparian forests in the lower Tarim River Basin

Evapotranspiration (ET) and its controlling mechanism over the desert riparian forests in arid regions are the important scientific basis for the water resources managements of the lower reaches of the inland rivers of China. Nearly three years of continuous measurements of surface ET, soil water content at different depths and groundwater table over a typical Tamarix spp. stand and a typical Populus euphratica stand were conducted in the lower reach of the Tarim River. The ET seasonal trends in the growing season were controlled by plant phenology, and ET in non-growing season was weak. The diurnal variations of ET resulting from the comprehensive effects of all atmospheric factors were significantly related with reference ET. The spatial pattern of ET was determined by vegetation LAI, more vegetation coverage, more ET amount. Groundwater is the water source of surface ET, and the soil water in shallow layers hardly took part in the water exchange in the groundwater-soil-plant-air system. The temporal processes of ET over the Tamarix stand and the Populus stand were similar, but the water consumption of the well-grown Populus euphratica was higher than that of the well-grown Tamarix spp. Further analysis indicates that plant transpiration accounts for most of the surface ET, with soil evaporation weak and negligible; groundwater table is a crucial factor influencing ET over the desert riparian forests, groundwater influences the processes and amounts of ET by controlling the growth and spatial distribution of desert riparian forests; quantifying the water stress of desert riparian forests using groundwater table is more appropriate, rather than soil water content. Based on the understanding of ET and water movements in the groundwater-soil-plant-air system, a generalized framework expressing the water cycling and its key controlling mechanism in the lower reaches of the inland rivers of China is described, and a simple model to estimate water requirements of the desert riparian forests is presented.

Total phosphorus concentrations in surface water of typical agro- and forest ecosystems in China, 2004–2010

The concentrations of total phosphorus (TP) from 83 surface water sampling sites in 29 of the Chinese Ecosystem Research Network (CERN) monitored ecosystems, representing typical agro- and forest ecosystems, were assessed using monitoring data collected between 2004 and 2010 from still and flowing surface water. Results showed that, TP concentrations were significantly higher in agro-ecosystems than those in forest ecosystems both for still and flowing surface water. For agroecosystems, TP concentrations in the southern area were significantly higher than those in the northern and northwestern areas for both still and flowing surface water, however no distinct spatial pattern was observed for forest ecosystems. In general, the median values of TP within agro- and forest ecosystems did not exceed the Class V guideline for still (0.2 mg·L−1) or flowing (0.4 mg·L−1) surface water, however, surface water at some agroecosystem sampling sites was frequently polluted by TP. Elevated concentrations were mainly found in still surface water at the Changshu, Fukang, Linze and Naiman monitored ecosystems, where exceedance (>0.2 mg·L−1) frequencies varied from 43% to 78%. For flowing water, elevated TP concentrations were found at the Hailun, Changshu and Shapotou monitored ecosystems, where exceedance (>0.4 mg·L−1) frequencies varied from 29% to 100%. Irrational fertilization, frequent irrigation and livestock manure input might be the main contributors of high TP concentrations in these areas, and reduced fertilizer applications, improvements in irrigation practices and centralized treatment of animal waste are necessary to control P loss in these TP vulnerable zones.

A Species-Specific and spatially-Explicit Model for Estimating Vegetation Water Requirements in Desert Riparian Forest Zones

Balancing human demands for water with environmental requirements to maintain functioning ecosystems requires the quantification of ecological water requirements. In arid regions, high spatial variability of vegetation cover and different water consumption of plant species make it different to estimate reasonable ecological water requirements. We developed a simple and practical approach that estimates the vegetation water requirements (VWRs) of desert riparian ecosystems. This model is species-specific and spatially-explicit; it considers the water consumption characteristics required by different species and highlights the impacts that high vegetation cover spatial variability has in arid regions on evapotranspiration. The model was parameterized based on the observation of the water consumption of two typical desert riparian species, Populus euphratica and Tamarix spp., in the lower basin of the Tarim River in northwestern China. Comparisons between the modeling results and measured data for two mature Populus and Tamarix stands indicate that the model is reasonable predictive. A case study in the lower basin of the Tarim River demonstrated the model’s practicality and transferability. This model could run based on near real-time or forest weather data and spatial vegetation patterns, and provides a continuous estimation of the temporal and spatial variations of the VWR. Particularly, this model forecasts VWRs under different vegetation spatial distribution and coverage scenarios, and evaluates the impacts and consequences of different management actions. This model can serve as a useful tool for management agencies interested in improving their decisions to allocate river water between human activities and natural ecosystems in arid regions.

Quantifying the impacts of river hydrology on riparian vegetation spatial structure: case study in the lower basin of the Tarim River, China

River hydrology largely determines the species composition and spatial distribution of riparian vegetation. However, it is difficult to determine the riparian vegetation spatial pattern that is only influenced by river hydrology due to the complex impacts of other influencing factors in field environments. We investigated the spatial structure of riparian vegetation in the lower basin of the Tarim River in hyper-arid western China, where distinct geomorphic and climatic features exclude the influences of factors other than river hydrological factors on the spatial structure of riparian vegetation. Landsat 8 OLI remotely sensed data were used to identify the different vegetation types and obtain the vegetation leaf area index (LAI) in the study area. Our results showed that the overall vegetation LAI exhibited a negative exponential declining trend with distance from the river, but this relationship was different for the different vegetation types, exhibiting a negative exponential relationship for the Populus woodlands and a Gaussian-type relationship for the Tamarix shrublands. The average width of the riparian vegetation decreased along the river by a negative power function. The average vegetation LAI decreased linearly along the river. The quantitative data we obtained regarding the river-induced variation in the riparian vegetation cover with the distance from the river channel would particularly be useful for modeling riparian vegetation dynamics and distributions.

Evaluation of non-water-stressed baselines for crop water stress monitoring in North China Plain

Canopy surface temperature measured with infrared thermometers or other remote IR sensor is an important tool for crop water stress monitoring. Crop Water Stress Index (CWSI) is the most often used index to detect crop water stress based on canopy surface temperature. The calculation of CWSI relies on two baselines: the non-water-stressed baseline, and the maximum stressed baseline. This paper evaluates the application of the most often used non-water-stressed baselines in North China Plain (NCP): the Idsos empirical baseline, and the Jacksons theoretical baseline. The results show that the Jacksons model makes better than the Idsos in monitoring winter wheat water stress in NCP.

Retrieval of directional fraction of vegetation cover using digital camera

A simple and feasible method to retrieve directional fraction of vegetation cover is presented. A digital camera is adopted to capture multi-angle images for a wheat field and trees. The viewing angles are controlled by an automatic multi-angle observation device. After image processing, directional fraction cover and gap probability can be obtained.