In searching for current topics related to LUCC and its consequences in hydrological regime changes, there seems to be a general trend in research papers arguing about "up-scaling¹". For example, In their invited commentary, Tetzlaff et al. (2007) argued that the progress toward a more holistic understanding of catchment functioning "has been hampered by a focus on reductionist approaches at small spatial² and temporal scales." (p1385). The authors then use a rather unusual but intriguing term to set the cornerstone of the needed research direction: CONNECTIVITY. Connectivity in the paper appears to carry the meaning of the flow between components, where components can be SNAP entities such as fishes, water, and catchment³. The authors also argued about the consideration of spatial patterns (and thus forming heterogeneous patches) in the ecohydrology research and how concepts of fragmentation⁴ (see Giambelluca, 2002) should be included to improve the understanding of hydro and ecological consequences of LUCC.

In general, I think the paper focuses on a better understanding of the forcing, i.e., what causes a certain phenomenon to occur, by anchoring the research on the concept of connectivity, or the flow channels between components in the physical systems. The focus is to examine how such channel respond or change at a larger scale.

The authors mentioned that the patches are dynamic (p1386) and are linked to hydrological connectivity. This somehow looks like a semantic net, implemented on a graph over time.



Footnotes
¹Scale means spatial extent and/or temporal duration. Here up-scaling means the incorporation of larger spatial extent (e.g., larger catchment areas) or longer time series data.
²Tetzlaff considers small scale as areas less than 10 square meters (called reach scale).
³I focus only on scientific definition of catchment. The separation of concept, meaning, and token (citation?) is not considered here.
⁴This is referred to as texture in remote sensing image interpretation. The concept seems to suggest that existing research either misses the linkage between patches by just sampling some representative patches and not considering the connections between them, or simply just neglect the existence of these patches by lumping them into a larger area with homogeneous properties.

Reference
Giambelluca, T. W., 2002, Hydrology of altered tropical forest, Hydrological Processes, 16(8), 1665-1669

Tetzlaff, D., Soulsby, C., Bacon, P. J., Yongson, A. F., Gibbins, C., and Malcolm, I. A., 2007, Connectivity between landscapes and riverscapes - a unifying theme in integrating hydrology and ecology in catchment science? Hydrological Processes,21(10), 1385-1389

The authors pointed out that extensive research was done on relating deforestation and hydrologic regime changes. However, they cited two problems of the existing research:

1. These works are mainly based on plots or small catchments of a few hectares (Bosch and Hewlett, 1982; Bruijnzeel, 1990; Bruijnzeel and Bremmer, 1989).
   
2. The results of the surface flow response to deforestation reported in various research are quite inconsistent.
   

The authors attempted to address is on the surface runoff and precipitation response for deforestation of large catchment (12100 km²)[link]. In general, this work operates at regional scale (large basin), and it attempts to address the scientific question of responses and impacts of LUCC in the (bio-)physical realm (using the NASA keyword). The study area is the Upper Nam Pong Basin. The period of analysis is between April 1969 - March 1995 (pp2737).

There appears to be a main issue that can be improved upon. The research looks at responses and impacts of LUCC on hydrologic regime, but there is not a feedback (Bronstert, 2004) from hydrologic regime to LUCC, which is something that can be examined further. There are also relatively minor issues such as the reason to use HBV model and the account of spatial variability by using the structure of basin-subbasin-elevation zone, but they are not the main concern at this point.


Reference
Bronstert, A, 2004, Rainfall-runoff modelling for assessing impacts of climate and land-use change, Hydrological Processes, 18(3), 567-570

Wilk, J., Andersson, L., and Plermkamon, V., 2001, Hydrological impacts of forest conversion to agriculture in a large river basin in northeast Thailand, Hydrological Processes, 15(14), 2729-2748 [doi]

The article by Yang et al., (2003) makes use of existing data (mainly from USGS) and estimation methods suggested from literature to estimate soil erosion at the global level. The estimation is based on Revised Universal Soil Loss Equation (RUSLE). The researchers simulated several scenarios from the past (1900s - 1980s), present (1980s) to the future (2090s, 2x of CO2) (see Table 2 of the article):

1. Scenario H: simulation based on historical land use and climate.
2. Scenario C: simulation based on present land use and climate.
3. Scenario D: simulation based on present land use and future climatic condition.
4. Scenario P: simulation based on potential vegetation under present climate.
5. Scenario F: simulation based on future land use.
6. Scenario E: simulation based on the future land use and predicted climatic condition.
   

The work on simulating various scenarios echoes with Bronstert's (2004) call for more work on simulation modeling.

This paper provides clear account on how global estimations were generated. The paper, however, seems to be vague about the contribution to our understandings of the "forcing" and how various components are interrelated to contribute to what we are experiencing right now. USing Bronstert's framework, this meanings that future research is needed to examine feedback between components of the system. It is also interesting to examine how the results match with estimation at the regional scale and explore the cause of the potential mismatches.

References
Bronstert, A, 2004, Rainfall-runoff modelling for assessing impacts of climate and land-use change, Hydrological Processes, 18, 567-570

Yang, D., Kanae, S., Oki, T., Koike, T., and Musiake, K., 2003, Global potential soil erosion with reference to land use and climate changes, Hydrological Processes, 17(14), 2913-2928 [doi]

The commentary of Bronstert (2004) cannot be labeled with NASA's LUCC scientific questions. The paper itself provides the research questions in the hydrological modeling domain with an emphasis on rainfall-runoff modeling. Several key factors that require special attention, as pointed out by the author, include (1) drivers, (2) appropriate spatiotemporal scales of observation¹, (3) incorporation of feedback, if necessary, (4) modeling aims (normal or extreme events), and (5) scenario-driving modeling.

The commentary provides a nice summary into six points. If interested, they can be found in the last page of the commentary.

Footnote
¹Aylward (2005) points out a related issue for appropriate spatiotemporal scales: most "existing studies have been undertaken at small scales (less than 10 km²) in headwater basins and over relatively short durations, making accurate extrapolation and 'upscaling' difficult.".

References
Aylward, B., 2005, Towards watershed science that matters, Hydrologic Processes, 19, 2643-2647
Bronstert, A, 2004, Rainfall-runoff modelling for assessing impacts of climate and land-use change, Hydrological Processes, 18, 567-570 [doi]