Browsing by Subject "Rainfall gradient"

Now showing 1 - 2 of 2

Feedbacks among chemical weathering, rock strength and erosion with implications for the climatic control of bedrock river incision
(2016-08) Murphy, Brendan Patrick, Ph. D.; Johnson, Joel P.; Gasparini, Nicole M; Mohrig, David; Breecker, Daniel O; Sklar, Leonard S
Understanding the processes that erode bedrock rivers and the factors that influence erosion rates is critical to predicting the feedbacks among climate, erosion and tectonics that drive the topographic evolution of unglaciated, active orogens. However, quantitative predictions for the coupling of these feedbacks are limited because the specific mechanisms by which climate controls erosion are poorly understood. Chemical weathering, a climate-dependent process, has been suggested to play a role in the erosion of bedrock rivers, but this idea has largely lacked supporting field or laboratory data. In this dissertation I present field data collected across an orographic precipitation gradient on the Kohala Peninsula of the Big Island of Hawaiʻi. This data demonstrates that the measured rock strength in bedrock river beds is a function of both climate-dependent chemical weathering and abrasional wear. Furthermore, accounting for the effect of chemical weathering on rock erodibility improves the predictions of long-term river profile evolution across Kohala Peninsula. Additional field data was collected and compared to previous experimental data of bedrock erosion in order to explore the feedbacks among chemical weathering, fluvial abrasion and topography at the scale of bed roughness. Finally, inspired by the findings of the field data, I developed a nonlinear dynamical model that finds quantifiable, predictive relationships for changes in rock strength as a function of chemical weathering and fluvial abrasion. The findings in this dissertation demonstrate that the erodibility of bedrock rivers can be influenced by the mechanism of climate-dependent chemical weathering, and that spatial distributions of rock erodibility that develop due to the interactions of chemical weathering and fluvial abrasion can influence the morphology of bedrock rivers from the scale of bed roughness to entire stream profiles.
Research on Texas savannas : fractional woody cover mapping, potential woody cover modelling, and woody plant encroachment analysis
(2017-12) Yang, Xuebin, Ph. D.; Crews, Kelley A.; Young, Kenneth R; Miller, Jennifer A; Arima, Eugenio Y; Huebner, Donald J
Tested in Texas savannas of a wide rainfall gradient, this dissertation endeavors to (1) map fractional woody cover at Landsat scale, for close and continuous woody plant encroachment monitoring, (2) model the pattern of potential woody cover over the present rainfall gradient, for implication of the end-point of woody plant encroachment, (3) analyze the rate and effect factors of woody plant encroachment under the regional context, for pertinent savanna management strategy. Web-Enabled Landsat Data (WELD) was used to calibrate the Salford Systems’ Classification and Regression Trees (CART) against training data of fractional woody cover derived from 1m resolution digital orthophotos. The CART model output was verified against an independent test data. This study provides a way to accurately monitor woody plant encroachment across savanna ecosystems at a fine spatial scale, and sets up a protocol for landscape components mapping at sub-pixel level in other ecosystems. The pattern of potential woody cover was modelled over the wide rainfall gradient at Landsat scale (30m) and MODIS scale (500m) respectively. While a positive linear relationship between potential woody cover and mean annual precipitation (MAP) was revealed at Landsat scale, a prominent three-segment relationship was observed at MODIS scale. This discrepancy corroborates the scale dependency of the primary determinants of savanna woody plant density. According to the three-segment pattern at MODIS scale, Texas savannas are divided into arid savanna (MAP < 600mm), semi-arid savanna (600mm < MAP < 735mm), and mesic savanna (MAP > 735mm). Analysis of the encroachment of Ashe juniper at its early life stage (initial ~20 years) at local (hectare) scale suggests that water availability has a significant positive effect on the encroachment rate in semi-arid savanna, but not in mesic savanna. In addition, a quadratic relationship was revealed between the encroachment rate and woody plant density in mesic savanna. That is, the encroachment rate increases with woody plant density by a threshold density, then starts decreasing with woody plant density. These results demonstrate that regional context such as rainfall and biological traits of woody species is critical to understand the trend of woody plant encroachment.