Toward Sustainability Assessment of Agricultural Ecosystem based on Thermodynamic Approach: A Case Study for Haenam Farmland in Korea

Abstract

An assessment of sustainability in an agricultural ecosystem is necessary to find out whether the current setting of the system under human intervention is a proper configuration toward sustainable management. This research attempted to utilize the long-term monitoring dataset of eddy covariance (EC) measurement in a typical agricultural ecosystem to quantify the ecosystem performance particularly in water use. The specific objectives were (1) to document decadal climatology, water use, energy, and carbon balance, and (2) to assess the state of this agricultural ecosystem based on thermodynamic perspective. The question is how to describe the current state of water use in an agricultural ecosystem and the dynamic under human management. This research was conducted by using eddy covariance measurement data for a decade in an agricultural ecosystem in Korea (Haenam Farmland in Korea, HFK). The mean annual precipitation (P) was 1454 ± 188 mm of which more than 53% occurred during the summer season. The mean annual downward shortwave radiation (Rs↓) was 5025 ± 154 MJ m-2 whereas that of air temperature (Ta) was 13.6 ± 0.1 ˚C with a gradually increasing pattern. Footprint climatology showed that most of the measured fluxes were from less than 200 meter around the tower. The Budyko curve indicated that the actual evapotranspiration (ET) is limited by the available energy. The annual ET was 639 ± 32 mm while the annual reference ET (ETo) was 728 ± 59 mm, resulting in an integrated crop coefficient (Kc) of 0.88 ± 0.1 for the rice growing season. The Kc value for initial stage was 0.87 ± 0.07, development stage 1.02 ± 0.08, middle stage 1.02 ± 0.08, and late stage 0.77 ± 0.10. The annual mean of inherent water use efficiency (Wei) was 16.4 ± 3.6 gC kg H2O-1 hPa with large interannual variations. In terms of the annual carbon budget, gross primary productivity (GPP), respiration of ecosystem (RE), and net ecosystem exchange (NEE) were 1235 ± 90, 1139 ± 54, and -97 ± 119 gC m-2, respectively. Annually integrated Rn was averaged to be 2567 ± 102 MJ m-2 and the energy partitioning in terms of the Bowen ratio was 0.39 ± 0.05. The energy balance ratio (EBR) for an annual budget closure ranged from 0.80 to 0.90. The mean annual internal entropy production (σ) was 12.88 ± 0.35 MJ m-2 K-1 while that of entropy transfer (J) was negative (-11.89 ± 0.36 MJ m-2 K-1), indicating the net transfer out of the system into the environment. The time rate of change in system entropy ( dS⁄dt ) fluctuated throughout the study period with an average of 1.39 ± 0.30 MJ m-2 K-1. The highlights of this research results are: 1) ET was limited not by the limitation of water but by the availability of energy, 2) the variation of Kc is mostly related to the fluctuation of ET, 3) low water use efficiency indicates a relatively poor use of water in this agricultural ecosystem, 4) the consistent overproduction of entropy throughout the decadal study period indicates a degradation of this agricultural ecosystem due to human disturbance, and 5) further studies are needed to bridge the quantified biophysical characteristics summarized above with thermodynamic and self-organization indicators tested in this study.