Demand for multi-scale weather data for regional crop modeling
Introduction
Crop models are becoming essential tools to study the interactions between plant, climate, soil and management practices in agricultural systems at different scales (Fischer et al., 2005, van Ittersum et al., 2008) to support policy and decision making for food security and land-use related management (De Vries et al., 1997, Rabbinge and Van Diepen, 2000). To apply crop models at regional scale, a level of homogeneity of spatial units or resolution needs to be assumed (Harrison et al., 2000), since the spatial heterogeneity of environmental conditions is ubiquitous (Wu, 2004). Once the resolution is determined, the input data such as climate and soil conditions, and management practices need to be aggregated or disaggregated to the same resolution to drive the crop models (Priya and Shibasaki, 2001). At a certain spatial extent, the resolution determines the detail in spatial variability that can be captured as well as the computational load. A fine resolution could capture more spatial detail, but it raises the demand for data storage, computational costs and efforts for results analysis (Zhao et al., 2013a). A coarse resolution requires less computing resources, but loses information in spatial variability.
The layers of coarse resolution are inherent or can be produced from layers of fine resolution by spatial aggregation (van Bussel et al., 2011). Spatial aggregation could be conducted by different methods such as spatial mean, median, and majority (Bian and Butler, 1999). No matter which method is adopted, loss of the spatial variability is inevitable (Li and Reynolds, 1995), because the resultant data uses one value to represent many others (Bian and Butler, 1999, Orcutt et al., 1968). The magnitude of the spatial variability loss varies with the features of the original data and the resolution differences. If the resultant area (also referred to as pixel) is homogeneous or covers a range of smaller pixels with similar values, the information loss would be minor. In contrast, the loss would be enormous if the covered smaller pixels are extremely heterogeneous. Here we term the variability and structure of the smaller pixels covered by the coarser resolution pixel as intra-pixel spatial heterogeneity. The loss of the intra-pixel spatial heterogeneity could be propagated by the crop model, thus affecting the simulated results such as crop yields (Guisan et al., 2007).
The effects of spatial resolution of weather data on simulated crop yields or crop phenology have been extensively studied (Angulo et al., 2013b, Folberth et al., 2012, Mearns et al., 1999, Mummery and Battaglia, 2002, van Bussel et al., 2011). These studies evaluated the effects by comparing the results simulated by input data at different spatial resolutions and found relatively small effects of weather data aggregation (Hansen and Jones, 2000, Nendel et al., 2013). Aggregation effects on mean values or medians are diminished when the response of the crop model to the change of weather data is close to linear or the non-linear response is not significant. This could result in a conclusion that minor aggregation effects are caused by input data aggregation.
The aim of spatially explicit modeling is capturing the spatial heterogeneity, which should be a priority in determining the spatial resolution (Di Vittorio and Miller, 2014). However, to our best knowledge no method has been developed to guide the choice of spatial resolution for regional scale simulation, because quantification of spatial heterogeneity of long-term time series of daily weather data is difficult. Weather data consisting of a set of variables, e.g. temperature, radiation, precipitation, and a high temporal frequency, often daily, is required for process-based crop models (Asseng et al., 2013, Balkovič et al., 2013). Within the spatial extent of a pixel (e.g. 10 km), the long-term weather condition is greatly influenced by the local topographic features (Ashcroft and Gollan, 2012). Topographic conditions are frequently used to correct the layers of weather data (Hancock and Hutchinson, 2006), thus long-term weather conditions are highly correlated with terrain features. Using the spatial heterogeneity of the topography to surrogate the weather conditions has the potential to bypass the difficulties in quantifying the spatial heterogeneity of different layers of weather variables (Kerr, 1993, Seoane et al., 2004). However, this has not been tested yet to inform about the required spatial resolution for simulating crop yield at regional scale using crop models. This paper aims to improve the understanding of the required spatial resolution of weather data for the simulation of regional crop yields and how the proper resolution can be determined.
Section snippets
General workflow of analysis
The general workflow of the simulation analysis is presented in Fig. 1. We produced 31-year weather data at 1 km resolution for the whole Germany and aggregated weather data into resolutions of 10, 25, 50, and 100 km. Using these data, we simulated a 30-year continuous cropping system of winter wheat, with a process-based crop model LINTUL2 (Light INTerception and UtiLization) (van Ittersum et al., 2003, van Oijen and Lefelaar, 2008) under rainfed condition. Three regions, respectively,
Model evaluation
The model overestimated the yields for the majority of the study area, especially in southern regions (Fig. 4b and c). The spatial patterns between observed and simulated yield were similar. Both the linear regression (r2) and correlation test (rho) showed that the model reproduced the temporal evolution quite well in the north-eastern regions, but relatively poor in the northern and south-western regions (Fig. 4d–g). The temporal variation of simulated yield was consistent with the historical
Model performance
The aim of this study was to evaluate the effect of weather data aggregation on crop yields simulated by a dynamic crop growth model. Specific emphasis was on the effects of aggregation on the spatial variability of simulated yields. Although we did not aim for a model evaluation of regional yield simulations, some demonstration of model performance for regional application was required to justify its use in the present study. The considered model LINTUL2 has been successfully applied in many
Conclusion
Regional scale application of crop models developed at field scale is becoming a common practice in impact assessment studies of climate variability and change. This paper evaluated the impact of weather data aggregation on simulation of rainfed crop yields at the national scale of Germany. It advances from early studies on aggregation by specifically addressing the effect of aggregation on spatial yield variability. Different resolutions of weather data simulated similar spatial patterns of
Acknowledgements
This study was jointly supported by the BMBF/BMELV project on “Modeling European Agriculture with Climate Change for Food Security (MACSUR)” (grant no. 2812ERA115) and the DFG projects on “Multi-scale modeling of the impacts of climate change and climate variability on winter wheat in Germany” (grant no. EW 119/5-1) and the Transregio (TRR32/2) on “Patterns in soil-vegetation-atmosphere systems: monitoring, modeling and data assimilation”. The comments from two anonymous reviewers greatly
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