Logging, erosion, and sediment yield in western China

We generally know that logging, and clear-cutting in particular, can cause enormous amounts of erosion due to loss of soil cohesion. So, one would predict that since most of the forests in western China were clear-cut in modern time, there should be a huge increase in erosion of soil from the hillslopes and sediment delivery to rivers (e.g., Yin and Li, 2001). Although most places in the world only monitor sediment concentration in rivers a few times a year and extrapolate the concentration for other days based on a sediment rating curve, the Chinese government has a large network of stations throughout the country that measure sediment concentrations and water discharge daily. So it turns out to be fairly easy to test the hypothesis that massive deforestation after the Communist government took power, called the Three Great Cuttings (the Great Leap Forward in the 1950s, the Grain as a Key Link policy in the 1970s, and Opening and Development in the 1980s), should show up in the sediment record for rivers in western China.

Taking advantage of this enormous and unique dataset, we calculated erosion rates for western China for the entire period of record of each of the gauging stations the Chinese set up. We also calculated values of possible parameters that could be related to erosion rates for each watershed, including mean local relief (a surrogate for slope steepness), rainfall, fraction farmland (a surrogate for development), and land cover. For each parameter we plotted the erosion rate for the basin as a function of the parameter value. We found that none of these parameters do a good job explaining the erosion rate patterns we see. Since we expect that some geomorphic or anthropogenic parameter will explain erosion rate patterns, this was rather surprising.

We then took the stations with at least 20 years of daily data and calculated annual erosion rates to see if there are any trends with time. We expected that erosion rates would increase just after each of the Three Great Cuttings. Once again, we saw no systematic trends in the erosion rates over time.

As a final test of possible effects of logging on sediment yield to rivers, we calculated sediment rating curves annually for each of the stations with 20+ years of daily data. A sediment rating curve is the best-fit line in log-log space of sediment concentration as a function of discharge. In disturbed basins the rating curve parameters (the slope and intercept of the line) often change (Environmental Protection Agency, 2006). We plotted the rating curve slope and intercept every year for each year of record. Once again we found that there seems to be no relationship between the rating curve parameters and any Chinese logging.

So what did we learn? We found that based on the sediment yield data we have, there seems to be no relationship between logging and sediment yield in western China. There are a few possible explanations for what we find. The first is that the logging didn't cause an increase in erosion. This could be because local people already were doing things, like farming, that caused erosion (e.g., Montgomery, 2007). Logging generally does cause an increase in erosion, at least over short periods of time, so this is not hugely likely. The second possibility is that the sediment just didn't make it to the rivers. If that is the case, then where did the sediment go?

• Environmental Protection Agency. (2006). Integration of Stream Stability, Reference Condition & Sediment Rating Curves. In "Watershed Assessment of River Stability & Sediment Supply." Environmental Protection Agency, url: http://www.epa.gov/warsss/sedsource/rivrelat.htm
• Henck, A., Montgomery, D. R., Huntington, K. W., and Liang, C. (2009). Cryptic legacy of Chinese Communist Party policies on sediment yield in SW China and SE Tibet. Geological Society of America Abstracts with Programs 41(7).
• Montgomery, D. R. (2007). Soil erosion and agricultural sustainability. Proceedings of the National Academy of Sciences of the United States of America 104(33), 13268-13272.
• Schmidt, A. H., Montgomery, D. R., Huntington, K. W., and Liang, C. (2011). The question of Communist land degradation: new evidence from local erosion and basin-wide sediment yield in SW China and SE Tibet. Annals of the Association of American Geographers 101(3), 1-19.
• Yin, H. F., and Li, C. G. (2001). Human impact on floods and flood disasters on the Yangtze River. Geomorphology 41(2-3), 105-109.