Jack Lewis

Comment on “Forest and floods: A new paradigm sheds light on age‐old controversies” by Younes Alila et al.

[1] The paper by Alila et al. [2009, hereafter referred to as AKSH] presents a technique for analyzing altered peak flow frequencies after logging. The paper suggests that the established method of chronologically pairing peak flows by corresponding hydrologic input at control and treated watersheds is inappropriate, leading to irrelevant research hypotheses and impeding scientific progress. In general, we agree that analyses of changes in flood frequency are useful for evaluating the effects of watershed disturbance, and that simple regression models often provide inadequate descriptions of posttreatment peak flow responses. However, the proposed method and accompanying discussion have several problems that undercut the strength of the paper’s conclusions: [2] 1. The recovery adjustment used by the method augments the effect the analysis is attempting to detect. [3] 2. Even in the absence of impacts, the frequency distribution of observed peaks is expected to have greater variance than that of predicted peaks, thereby introducing an artificial shift when comparing upper quantiles of observed and expected frequency distributions. [4] 3. A more appropriate analysis of uncertainty is needed if the utility of the method is to be validly assessed. [5] 4. Frequency pairing does not overcome the problem of low power in testing for changes in very large events prior to forest regrowth. [6] 5. The relative merits of alternative statistical approaches are mischaracterized.

Temporal and Spatial Trends in Nutrient and Sediment Loading to Lake Tahoe, California‐Nevada, USA

Since 1980, the Lake Tahoe Interagency Monitoring Program (LTIMP) has provided streamdischarge and water quality data—nitrogen (N), phosphorus (P), and suspended sediment—at more than 20 stations in Lake Tahoe Basin streams. To characterize the temporal and spatial patterns in nutrient and sediment loading to the lake, and improve the usefulness of the program and the existing database, we have (1) identified and corrected for sources of bias in the water quality database; (2) generated synthetic datasets for sediments and nutrients, and resampled to compare the accuracy and precision of different load calculation models; (3) using the best models, recalculated total annual loads over the period of record; (4) regressed total loads against total annual and annual maximum daily discharge, and tested for time trends in the residuals; (5) compared loads for different forms of N and P; and (6) tested constituent loads against land use-land cover (LULC) variables using multiple regression. The results show (1) N and P loads are dominated by organic N and particulate P; (2) there are significant long-term downward trends in some constituent loads of some streams; and (3) anthropogenic impervious surface is the most important LULC variable influencing water quality in basin streams. Many of our recommendations for changes in water quality monitoring and load calculation methods have been adopted by the LTIMP. (KEY TERMS: biogeochemistry; environmental impacts; lakes; rivers/streams; monitoring; statistics; environmental sampling; nutrients; sediment; eutrophication; total load.) Coats, Robert, Jack Lewis, Nancy Alvarez, and Patricia Arneson, 2016. Temporal and Spatial Trends in Nutrient and Sediment Loading to Lake Tahoe, California-Nevada, USA. Journal of the American Water Resources Association (JAWRA) 1-19. DOI: 10.1111/1752-1688.12461

Turbidity Responses from Timber Harvesting, Wildfire, and Post-Fire Logging in the Battle Creek Watershed, Northern California

The Battle Creek watershed in northern California was historically important for its Chinook salmon populations, now at remnant levels due to land and water uses. Privately owned portions of the watershed are managed primarily for timber production, which has intensified since 1998, when clearcutting became widespread. Turbidity has been monitored by citizen volunteers at 13 locations in the watershed. Approximately 2000 grab samples were collected in the 5-year analysis period as harvesting progressed, a severe wildfire burned 11,200 ha, and most of the burned area was salvage logged. The data reveal strong associations of turbidity with the proportion of area harvested in watersheds draining to the measurement sites. Turbidity increased significantly over the measurement period in 10 watersheds and decreased at one. Some of these increases may be due to the influence of wildfire, logging roads and haul roads. However, turbidity continued trending upwards in six burned watersheds that were logged after the fire, while decreasing or remaining the same in two that escaped the fire and post-fire logging. Unusually high turbidity measurements (more than seven times the average value for a given flow condition) were very rare (0.0% of measurements) before the fire but began to appear in the first year after the fire (5.0% of measurements) and were most frequent (11.6% of measurements) in the first 9 months after salvage logging. Results suggest that harvesting contributes to road erosion and that current management practices do not fully protect water quality.