农地保护的外部效益测算——选择实验法在武汉市的应用

农地保护的外部效益是农地保护补偿政策制定的依据.对外部效益进行了理论界定和实证测算,认为农地保护的外部效益是农村集体和农地使用者以外个体从农地保护中得到的收益,通常是非市场收益,对其测算应区分外部效益的内部(外部效益发出者)和外部(外部效益接受者),并确定外部区域的范围.实证利用选择实验模型对武汉市5个区域进行了支付意愿测算,在内外部划分的基础上估算了武汉市农地保护的外部效益并比较了不同方法下外部效益测算结果的差别.结果表明,武汉市耕地、园地、林地和农村水面的外部效益分别为30773.2,653860.0,119267.0和82472.7元/hm2；用行政区域直接代替外部性影响区域的做法会使耕地外部效益的测算结果偏高,也会造成园地、林地和农村水面测算的误差,为避免这类误差,外部性测算及相关补偿、税费政策应考虑内外部区域的划分.

Measuring external benefits of agricultural land preservation: an application of choice experiment in Wuhan, China

External benefits of agricultural land preservation are essential for the agricultural land preservation compensation and the decision-making of rural-urban land conversion. However, estimating external benefits of agricultural land preservation faces two challenges. One is the proper identification of the influence extent of external benefits; the other the explicit measurement of the external benefits. While previous literatures focused on external benefits of agricultural land preservation in the term of environmental and social effects, few studies analyze the influence extent of external benefits and the method of external benefits measurement. This paper tries to fill this research gap. We defined and identified the external benefits extent of agricultural land preservation and measured external benefits of land preservation basing on investigation of WTPs(willingness to pay) for agricultural land preservation in Wuhan, China. First, we theoretically defined and identified influence extent of external benefits of agricultural land preservation. Based on Buchanan and Stubblebine''s definition of externality, external benefits of agricultural land preservation in China are not being rural collective and farmers, but individual utilities from agricultural land preservation. From that definition, theoretical analysis showed that the measurement process should distinguish internal and external parts (generator and receiver). After Loomis (2000), we found that willingness to pay decreases with distance and regarded boundaries of extent as where WTP is equal to 0. However, the linear relationship between WTP and distance is not significant and the scale of agricultural land preservation is county in China. So we further modified Loomis'' method to fit into our China case study. Second, we estimated external benefits of agricultural land preservation empirically basing on WTP method and extent identification of external benefits. In the WTP estimation process, CE (choice experiment) method was applied where farmland, garden land, forest and fishing pond preservation were chosen as attribute variables. As a result, the mean WTPs of farmland, garden land, forest and fishing pond are 257.69, 311.31, 333.81 and 129.28 Yuan per household respectively. These results are similar with those by Cai''s(2007) using CVM(contingent valuation method). In the extent identification process, stated preference method was used. We found that most residents from central districts of Wuhan chose to preserve agricultural land in the whole Wuhan city,while residents from districts which still have lots of farmlands intended to pay for preserving agricultural land in their own district. After WTP estimate and extent identification, three methods were proposed to measure the external benefits of agricultural land preservation. We further analyzed the errors among different extent definitions. The first method, which is traditional, took administrative jurisdiction as extent of external part; the second method defined extent of external part as all the places where residents have the highest support; the third, which is close to the reality, considered all individuals'' preference of preservation extent. Our third method shows that the external benefits of farmland, garden land, forest and fishery pond in Wuhan are 30773.2yuan/hm²,653860.0yuan/hm²,119267.0yuan/hm² and 82472.7yuan/hm² respectively. These values are significantly different than those from the first and second methods. Apparently, using jurisdiction extent instead of extent of external part would overstate external benefits of agricultural land. We further discussed the policy implications from such different estimates.