Relating land cover to stream properties in southern Chilean watersheds: trade-off between geographic scale, sample size, and explicative power

Abstract

Several studies relating land cover to stream properties have used sample sizes of more than 100 watersheds, but the variance that they explain is moderate to low (R 2 less than 50%), limiting the predictive value of these studies when their models are applied to watersheds that were not included in the models' development. We hypothesize that this is due to the increases in variation that occur with increases in sample size and in the geographic scales of the areas in which the watersheds are distributed. Land cover alone cannot explain all of that variation; more predictors must be considered. Conversely, models with high explicative power would require relatively small sample sizes distributed over small areas. This hypothesis was evaluated sampling 17 watersheds from southern Chile's Lake Region, for which we developed regressive models between land cover/ watershed area/precipitation/geomorphology and stream properties (i.e., conductivity, temperature). With a maximum n = 15 watersheds, on a regional scale, a poorly explained variation in hydrologic variables (mean 37-49%) was obtained. The R 2 increased slightly, to 45-52%, when precipitation was included as a predictor. In half of the cases analyzed, the models improved when geomorphology was considered as an additional predictor (60-66%), supporting our hypothesis. Furthermore, when our analysis was restricted to a narrower latitudinal span (n = 9), the R 2 was much stronger (68-87%) when only land cover and watershed area were included as predictors. These percentages also increased when more predictors were incorporated. Nevertheless, a portion of unexplained variance remained that would require the consideration of more predictors, such as geology and edaphology. The documented tradeoff provides evidence that argues against the spatial generality of land cover/stream property models.

Ecohidrology and Nutrient Fluxes in Forest Ecosystems of Southern Chile

“…In our study, the differences in DIN retention were evident between native forests and Eucalyptus plantation, as also has been described previously by [12]. However, [35] observed using land cover, watershed area and precipitation as predictors for water quality (nitrate, ammonia, DON, TDP and electric conductivity) for local models explained 79.5% of the variance. …”

Buffer effects of streamside native forests on water provision in watersheds dominated by exotic forest plantations

“…Numerous studies have shown that variations in nutrient and sediment concentration and export in stream water are a function of changes in forest land cover, watershed properties, and precipitation (e.g. Graham, ; Strayer et al ., ; Cuevas et al ., ; Little et al ., ). The importance of forest stream buffers to conserve stream water quality has been recognized in North America and Europe for several decades, but much of the work on dissolved N and forest buffers has been performed in N‐saturated systems (Osborne and Kovacic, ).…”

Throughfall enrichment and stream nutrient chemistry in small headwater catchments with different land cover in southern Chile

“…These changes may affect the abundance of aerobic and anaerobic microorganisms and subsequently affect the cycling of several elements, including N (Huygens & Boeckx, ). However, Cuevas et al () observed that land cover and watershed area explained nearly 90% of the variability in water quality (NO 3 − − N, NH 4 + − N, DON, Total‐P, and electric conductivity) using local models on dry season. However, during wet season, land cover and watershed area just explained 70%, and with the addition of geomorphology and precipitation, it explained 85%.…”

“…Water quality control is recognized as one of the main ecosystem services provided by forests (Alexander, Boyer, Smith, Schwarz, & Moore, ). Several authors have found that different land cover (Cuevas et al, ; Oyarzún, Aracena, Rutherford, Godoy, & De Schrijver, ; Vitousek et al ., ), forest type (De Schrijver et al, ), and hydrology (Campbell et al . , ) affect N and P cycling in forests and have a direct impact on nutrient exports to streams.…”