Abstract
Governments around the world are increasingly invoking hydrological services, such as flood mitigation and water purification, as a justification for forest conservation programs in upstream areas. Yet, rigorous empirical evidence that these programs are actually delivering the intended services remains scant. We investigate the effect of deforestation on flood-mitigation services in Peninsular Malaysia during 1984–2000, a period when detailed data on both flood events and land-use change are available for 31 river basins. Floods are the most common natural disaster in tropical regions, but the ability of tropical forests to mitigate large-scale floods associated with heavy rainfall events remains disputed. We find that the conversion of inland tropical forests to oil palm and rubber plantations significantly increased the number of days flooded during the wettest months of the year. Our results demonstrate the importance of using disaggregated land-use data, controlling for potentially confounding factors, and applying appropriate estimators in econometric studies on forest ecosystem services.
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Notes
A pioneering cross-country study on the economics of natural disasters by Kahn (2005) included an analysis of flood occurrence, but it focused on issues other than the effects of land use. It obtained flood data from the Emergency Events Database (EM-DAT; http://www.emdat.be), which is widely used but has shortcomings that include separate flood events being recorded as a single one and smaller flood events being underreported in developing countries (Jonkman 2005, p. 153).
The other two are the states of Sabah and Sarawak on Borneo.
These surveys do not cover Sabah and Sarawak.
If our objective had been to investigate underlying hydrological processes, then we would have needed to include potentially confounding factors directly as covariates instead of using fixed effects to sweep away their effects.
One type of overfitting in panel models occurs when fixed effects for cross-sectional units are included at a level that is more finely disaggregated than necessary for identifying the effect of interest; it causes standard errors to be underestimated but does not affect the consistency of parameter estimates (Ritschl 2009). In our models, this type of overfitting would occur if RBMUs could be classified into homogeneous groups, in which case fixed effects should be included for groups instead of individual RBMUs. The RBMUs do not fall into obvious groups, however, as they differ greatly in area, topography, and other fixed characteristics. Moreover, this type of overfitting occurs only if the number of fixed effects is large relative to the number of observations, which was not the case in our models (31 RBMU fixed effects, 636–716 observations).
Including a large number of controls can also cause multicollinearity, but given our objective this was a lesser evil than omitted-variables bias. Multicollinearity inflates standard errors, but it does not bias parameter estimates (Kennedy 2002, pp. 193–194). Hence, it contributes to a conservative estimation strategy by reducing the risk of overestimating parameter significance.
Similarly, given that the Department of Agriculture conducts the land-use surveys using the same procedures in all parts of Peninsular Malaysia, there is no apparent reason to expect measurement error in the 1984–1985, 1997–1998, and 2004–2005 surveys to be nonrandom across RBMUs or land uses.
Clustering is a second reason that we did not use the AIC or BIC to guide model specification: these statistics should not be used when data have a clustered structure (Hilbe 2011, p. 69).
The 10-month gap from 1 year to the next meant that we did not need to cluster the standard errors at an even lower frequency, e.g., the entire sample period for a given RBMU, because the gap interrupted the serial correlation process between years.
Descriptive statistics differed little for the 716-observation sample used in the models for number of floods.
The parameter estimates cannot be directly compared, as they are interpretable as semi-elasticities in the Poisson model and marginal effects on log odds ratios in the logit model.
These effects refer to months when flooding occurred, not all months in the sample, as they are based on coefficients from the zero-truncated component of the hurdle model.
One RBMU contained a much higher percentage of wetland forest than the others: 59 % as of 1984, with the next highest percentage being 32 %. Results changed little if we excluded that RBMU from the sample. For example, the coefficients on oil palm, rubber, and wetland forest in model (3) when estimated for that sample were 0.280 (\(P =\) 0.001), 0.275 (\(P =\) 0.002), and 0.312 (\(P =\) 0.003).
We also considered the effect of using samples that defined wet months in ways other than the core wet season. In one variant, we expanded the sample by extending the wet season to include the preceding and following months. One would expect this to reduce the absolute value of the regression coefficients on the land-use variables, given that the coefficients reflect mean monthly effects on days flooded and that extending the sample causes those effects to be averaged over a period that includes months when there is less rainfall, less saturated soils, and thus less flooding. (In the extreme case of no flooding at all, the effects of land use on days flooded during those months would necessarily be zero.) This is what we found; for example, the coefficients on oil palm and rubber were reduced to 0.147 and 0.155, respectively (\(P < 0.05\) for both). In a second variant, we ignored seasonality and included months with rainfall in the 70th percentile. This yielded a sample with 670 observations, similar to the samples in Table 3. Coefficients on the land-use variables retained their signs but became more significant and smaller: the positive ones became less positive, and the negative one became more negative. The coefficients on oil palm and rubber were 0.192 and 0.212, respectively (\(P < 0.001\) for both).
Flooding could be affected by time-varying factors other than the three included in model (3). One example is regional income; another is forest management, which under the Malaysian constitution is the responsibility of state governments. To investigate the influence of such factors, we added annual trends for the nine states in the sample to model (3). With the exception of the “other” land-use variable, which remained negative and insignificant, coefficients on all the other land-use variables increased. For example, the coefficients on oil palm and rubber were 0.385 and 0.602, respectively (\(\hbox {P} < 0.01\) for both).
\(2 \times (0.268 \times 1.11) / (0.01 \times 3343) = 0.018\).
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Acknowledgments
This study was funded by the Global Environment Facility through the United Nations Development Programme (MAL/04/G31). Additional support was provided by the Government of Malaysia, through the Ministry of Natural Resources and Environment and the Forest Research Institute Malaysia, and by the Center for International Forestry Research. The cooperation of the Departments of Agriculture, Drainage and Irrigation, and Statistics of the Government of Malaysia is gratefully acknowledged as are helpful comments by M Jeuland, D Richter, and E Sills.
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Tan-Soo, JS., Adnan, N., Ahmad, I. et al. Econometric Evidence on Forest Ecosystem Services: Deforestation and Flooding in Malaysia. Environ Resource Econ 63, 25–44 (2016). https://doi.org/10.1007/s10640-014-9834-4
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DOI: https://doi.org/10.1007/s10640-014-9834-4