Earth Sciences Commons^™

Potential Of Using Spectral Vegetation Indices For Corn Green Biomass Estimation Based On Their Relationship With The Photosynthetic Vegetation Sub-Pixel Fraction, Luan Peroni Venancio, Everardo Chartuni Mantovani, Cibele Hummel Do Amaral, Christopher M.U. Neale, Ivo Zution Gonçalves, Roberto Filgueiras, Fernando Coelho Eugenio

Daugherty Water for Food Global Institute: Faculty Publications

Crop biomass (Bio) is one of the most important parameters of a crop, and knowledge of it before harvest is essential to help farmers in their decision making. Both green and dry Bio can be estimated from vegetation spectral indices (VIs) because they have a close relationship with accumulated absorbed photosynthetically active radiation (APAR), which is proportional to total Bio. The aims of this study were to analyze the potential capacity of spectral vegetation indices in estimating corn green biomass based on their relationship with the photosynthetic vegetation sub-pixel fraction derived from spectral mixture analysis and to analyze the best …

Ferrihydrite Reduction Increases Arsenic And Uranium Bioavailability In Unsaturated Soil, Arindam Malakar, Michael Kaiser, Daniel D. Snow, Harkamal Walia, Banajarani Panda, Chittaranjan Ray

Daugherty Water for Food Global Institute: Faculty Publications

Redox driven mobilization and plant uptake of contaminants under transiently saturated soil conditions need to be clarified to ensure food and water quality across different irrigation systems. We postulate that solid-phase iron reduction in anoxic microsites present in the rhizosphere of unsaturated soil is a key driver for mobilization and bioavailability of contaminants under nonflooded irrigation. To clarify this, two major crops, corn and soybean differing in iron uptake strategies, were grown in irrigated synthetic soil under semiarid conditions with gravimetric moisture content ∼12.5 ± 2.4%. 2-line ferrihydrite, which was coprecipitated with uranium and arsenic, served as the only iron …

Use Of Multiple Environment Variety Trials Data To Simulate Maize Yields In The Ogallala Aquifer Region: A Two Model Approach, Vaishali Sharda, Mesfin Mekonnen, Chittaranjan Ray, Prasanna H. Gowda

Daugherty Water for Food Global Institute: Faculty Publications

With a long-term goal to optimize use of groundwater in the Ogallala Aquifer Region (OAR) to sustain food production systems, this study was conducted to calibrate Decision Support System for Agrotechnology Transfer (DSSAT) and AquaCrop crop modeling platforms to simulate maize production at a regional scale using historic datasets. Calibration of the models with local crop growth data and crop management practices is important, but usually this in-season crop growth information is not available. This study determined the possibility of using maize variety trial data for the evaluation of the CSM-Crop Estimation through Resources and Environmental Synthesis-Maize and AquaCrop models …

Hydrogeology And Geology Of The Lower North And South Platte Rivers Of Nebraska: Nebraska Geological Society Field Trip 2020, Douglas R. Hallum, Paul R. Hanson, Shane Tucker, Robert F. Diffendal Jr.

Robert F. Diffendal, Jr., Publications

The 2020 Nebraska Geological Society field trip will visit central Nebraska, and explore the geology and hydrogeology of the upper Platte, Lower South Platte and Lower North Platte rivers. The field trip area consists of a roughly east-west strip of land, approximately 100 kilometers long, and generally less than 10 kilometers north and south of the Platte, North Platte and South Platte rivers’ channels over most of the reach; from Brady, NE, upstream to Big Springs on the South Platte River and Lewellen on the North Platte River (Map 1). In addition to the river valleys, participants will observe alluvial …

Measuring The Occurrence Of Antibiotics In Surface Water Adjacent To Cattle Grazing Areas Using Passive Samplers, Nasrin Naderi Beni, Daniel D. Snow, Elaine D. Berry, Aaron R. Mittelstet, Tiffany Messer, Shannon L. Bartelt-Hunt

Nebraska Water Center: Faculty Publications

A wide variety of antibiotics and other pharmaceuticals are used in livestock production systems and residues passed to the environment, often unmetabolized, after use and excretion. Antibiotic residuesmay be transported frommanure-treated soils via runoff and are also capable of reaching surface and groundwater systems through a variety of pathways. The occurrence and persistence of antibiotics in the environment is a concern due to the potential for ecological effects and proliferation of environmental antibiotic resistance in pathogenic organisms. In the present study, the occurrence and seasonal variation of 24 commonly-used veterinary antibiotics was evaluated in surface water adjacent to several livestock …

Critical Analysis Of The Value Of Drought Information And Impacts On Land Management And Public Health, Tingting Liu, Kelly Helm Smith, Richard Krop, Tonya Haigh, Mark Svoboda

National Drought Mitigation Center: Faculty Publications

This paper reviews previous efforts to assign monetary value to climatic or meteorological information, such as public information on drought, climate, early warning systems, and weather forecast information. Methods and tools that have been explored to examine the benefits of climatic and meteorological information include the avoided cost, contingent valuation, choice experiments, benefit transfer, and descriptive approaches using surveys. The second part of this paper discusses specific considerations related to valuing drought information for public health and the Bureau of Land Management. We found a multitude of connections between drought and the land management and health sectors in the literature. …

Using Climate To Explain And Predict West Nile Virus Risk In Nebraska, Kelly Smith, Andrew Tyre, Jeff Hamik, Michael Hayes, Yuzhen Zhou, Li Dai

National Drought Mitigation Center: Faculty Publications

We used monthly precipitation and temperature data to give early warning of years with higher West Nile Virus (WNV) risk in Nebraska. We used generalized additive models with a negative binomial distribution and smoothing curves to identify combinations of extremes and timing that had the most influence, experimenting with all combinations of temperature and drought data, lagged by 12, 18, 24, 30, and 36 months. We fit models on data from 2002 through 2011, used Akaike's Information Criterion (AIC) to select the best‐fitting model, and used 2012 as out‐of‐sample data for prediction, and repeated this process for each successive year, …

David Quentin Bowen: A Memorial, John T. Andrews, David R. Bridgland, Peter U. Clark, Thomas M. Cronin, Claude Hillaire-Marcel, Darrel Maddy, Gifford H. Miller, Daniel R. Muhs, Colin V. Murray-Wallace

United States Geological Survey: Staff Publications

The Quaternary community lost a giant and a leader on October 5, 2020, when David Quentin Bowen, known to many as “DQ” and founding editor of Quaternary Science Reviews, passed away in Cardiff. Born on February 14, 1938 in Llanelli, SouthWales, he received his PhD at University College London. David’s 50 years of contributions to our science cannot be adequately summarized in a brief memorial but past, present, and future generations of Quaternary scientists will long remember his landmark achievements in publishing, his scientific contributions, and his personal and professional class in all his endeavors.

Testing Glacial Isostatic Adjustment Models Of Last-Interglacial Sea Level History In The Bahamas And Bermuda, Daniel R. Muhs, Kathleen R. Simmons, R. Randall Schumann, Eugene S. Schweig, Mark P. Rowe

United States Geological Survey: Staff Publications

Part of the spatial variation in the apparent sea-level record of the last interglacial (LIG) period is due to the diverse response of coastlines to glacial isostatic adjustment (GIA) processes, particularly where coastlines were close to the Laurentide Ice Sheet during the past two glacial periods. We tested modeled LIG paleo-sea levels on New Providence Island (NPI), Bahamas and Bermuda by investigating emergent coral patch reefs and oolitic/peloidal beach deposits. Corals with closed-system histories collected from patch reefs on NPI have ages of 128-118 ka and ooids/peloids from beach ridges have closed-system ages of 128-116 ka. Elevations of patch reefs …

Uptake, Metabolism, And Elimination Of Fungicides From Coated Wheat Seeds In Japanese Quail (Coturnix Japonica), Michael S. Gross, Thomas G. Bean, Michelle L. Hladik, Barnett A. Rattner, Kathryn M. Kuivila

United States Geological Survey: Staff Publications

Pesticides coated to the seed surface potentially pose an ecological risk to granivorous birds that consume incompletely buried or spilled seeds. To assess the toxicokinetics of seeds treated with current-use fungicides, Japanese quail ( Coturnix japonica) were orally dosed with commercially coated wheat seeds. Quail were exposed to metalaxyl, tebuconazole, and fludioxonil at either a low dose (0.0655, 0.0308, and 0.0328 mg/kg of body weight, respectively) or a high dose (0.196, 0.0925, and 0.0985 mg/kg of body weight, respectively). Fungicides were rapidly absorbed and distributed to tissues. Tebuconazole was metabolized into tert-butylhydroxy-tebuconazole. All compounds were eliminated to below detection limits …

Adjusting The Lens Of Invasion Biology To Focus On The Impacts Of Climate-Driven Range Shifts, Piper D. Wallingford, Toni Lyn Morelli, Jenica M. Allen, Evelyn M. Beaury, Dana M. Blumenthal, Bethany A. Bradley, Jeffrey S. Dukes, Regan Early, Emily J. Fusco, Deborah E. Goldberg, Inés Ibáñez, Brittany B. Laginhas, Montserrat Vilà, Cascade J.B. Sorte

United States Geological Survey: Staff Publications

As Earth’s climate rapidly changes, species range shifts are considered key to species persistence. However, some range-shifting species will alter community structure and ecosystem processes. By adapting existing invasion risk assessment frameworks, we can identify characteristics shared with high-impact introductions and thus predict potential impacts. There are fundamen- tal differences between introduced and range-shifting species, primarily shared evolutionary histories between range shifters and their new community. Nevertheless, impacts can occur via analogous mechanisms, such as wide dispersal, community disturbance and low biotic resistance. As ranges shift in response to climate change, we have an opportunity to develop plans to facilitate …

Trends In Streamflow And Precipitation For Selected Sites In The Elkhorn River Basin And In Streamflow In The Salt Creek And Platte River Basins, Nebraska, 1961–2011, Benjamin J. Dietsch, Kellan R. Strauch

United States Geological Survey: Publications

To better understand the streamflow trends at the streamgages in the Elkhorn River Basin in Nebraska, the U.S. Geological Survey (USGS) in cooperation with the Lower Elkhorn Natural Resources District further investigated streamflow trends at the eight streamgages on the Elkhorn River, Salt Creek, and the Lower Platte River that indicated a positive trend in streamflow characteristics and analyzed precipitation trends in the four basins upstream from the Elkhorn River Basin streamgages. An analysis of four streamgages in the Elkhorn River Basin, one streamgage in Salt Creek Basin, and three streamgages in the Lower Platte River Basin that had previously …

Modeling Escherichia Coli In The Missouri River Near Omaha, Nebraska, 2012–16, Brenda K. Densmore, Brent M. Hall, Matthew T. Moser

United States Geological Survey: Publications

The city of Omaha, Nebraska, has a combined sewer system in some areas of the city. In Omaha, Nebr., a moderate amount of rainfall will lead to the combination of stormwater and untreated sewage or wastewater being discharged directly into the Missouri River and Papillion Creek and is called a combined sewer overflow (CSO) event. In 2009, the city of Omaha began the implementation of their Long Term Control Plan (LTCP) to mitigate the effects of CSOs on the Missouri River and Papillion Creek. As part of the LTCP, the city partnered with the U.S. Geological Survey (USGS) in 2012 …

Annotated Bibliography Of Scientific Research On Greater Sage-Grouse Published From 2015 To 2019, Sarah K. Carter, Robert S. Arkle, Heidi L. Bencin, Benjamin R. Harms, Daniel J. Manier, Aaron N. Johnston, Susan L. Phillips, Steven E. Hanser, Zachary H. Bowen

United States Geological Survey: Publications

The greater sage-grouse (Centrocercus urophasianus; hereafter GRSG) has been a focus of scientific investigation and management action for the past two decades. The 2015 U.S. Fish and Wildlife Service listing determination of “not warranted” was in part due to a large-scale collaborative effort to develop strategies to conserve GRSG populations and their habitat and to reduce threats to both. New scientific information augments existing knowledge and can help inform updates or modifications to existing plans for managing GRSG and sagebrush ecosystems. However, the sheer number of scientific publications can be a challenge for managers tasked with evaluating and …

Groundwater Availability Of The Northern High Plains Aquifer In Colorado, Kansas, Nebraska, South Dakota, And Wyoming, Steven M. Peterson, Jonathan P. Traylor, Moussa Guira

United States Geological Survey: Publications

The Northern High Plains aquifer underlies about 93,000 square miles of Colorado, Kansas, Nebraska, South Dakota, and Wyoming and is the largest subregion of the nationally important High Plains aquifer. Irrigation, primarily using groundwater, has supported agricultural production since before 1940, resulting in nearly $50 billion in sales in 2012. In 2010, the High Plains aquifer had the largest groundwater withdrawals of any major aquifer system in the United States.Nearly one-half of those withdrawals were from the Northern High Plains aquifer, which has little hydrologic interaction with parts of the aquifer farther south. Land-surface elevation ranges from more than 7,400 …

Biotic And Abiotic Factors Influencing Arsenic Biogeochemistry And Toxicity In Fluvial Ecosystems: A Review, Laura Barral-Fraga, María Teresa Barral, Keeley L. Macneill, Diego Martiñá-Prieto, Soizic Morin, María Carolina Rodríguez-Castro, Baigal-Amar Tuulaikhuu, Helena Guasch

School of Natural Resources: Faculty Publications

This review is focused on the biogeochemistry of arsenic in freshwaters and, especially, on the key role that benthic microalgae and prokaryotic communities from biofilms play together in through speciation, distribution, and cycling. These microorganisms incorporate the dominant iAs (inorganic arsenic) form and may transform it to other arsenic forms through metabolic or detoxifying processes. These transformations have a big impact on the environmental behavior of arsenic because different chemical forms exhibit differences in mobility and toxicity. Moreover, exposure to toxicants may alter the physiology and structure of biofilms, leading to changes in ecosystem function and trophic relations. In this …

2020 Sponsorship Opportunities. Lincoln, Neb., Usa, Oct. 7-9, 2020

Daugherty Water for Food Global Institute: Administrative Materials

The Water for Food Global Conference convenes leading international experts and organizations to focus on achieving global water and food security. “Cultivating Innovation: The Next Decade,” will include keynote speakers, sessions, interactive learning and networking opportunities. The conference also celebrates the 10 year anniversary of the Daugherty Water for Food Global Institute, headlining a week of water and food security-related activities.

Topics include: • Research and Technology Innovation • Policy/Management Tools and Best Practices • Capacity Development/Youth and Students • Water Quality and Public Health • Climate Change and Ecosystems • Women for Water and Food • Nutrition and Health …

Ashfall Fossil Beds: From Waterhole To Rhino Barn, Sandy Mosel

University of Nebraska State Museum: Programs Information

Twelve million years ago a volcanic super-eruption in what is now southwestern Idaho created an enormous ashfall that blanketed the Great Plains with several inches of volcanic ash, devastating the local wildlife. Evidence of this geologic event lies within the hills of the Niobrara River Valley in northeast Nebraska, at a place aptly named the Ashfall Fossil Beds State Historical Park. Located near the tiny village of Royal, the Ashfall Park has been developed around a waterhole where animals succumbed after breathing in the volcanic dust. Buried deeply by windblown volcanic ash in the weeks following the eruption, the skeletons …

Diversity And Equality In Honours And Awards Programs – Steps Towards A Fair Representation Of Membership, Mary Anne Holmes, Latoya Myles, Blair Schneider

Department of Earth and Atmospheric Sciences: Faculty Publications

Honours and awards bestowed by professional societies recognize and reward members who have advanced the goals and values of that society. All too often, however, awards reflect a small network of people who know about the awards and participate in the process. This network works wonderfully for the people lucky enough to be in it, but typically neglects the full range and breadth of scholarship and service within the society. We represent a combined 15C years’ experience on the honours’ committee for a large professional society (the American Geophysical Union) and here offer strategies to increase the representation of honourees.Women …

A Model Of Icewedge Polygon Drainage In Changing Arctic Terrain, Vitaly Zlotnik, Dylan R. Harp, Elchin E. Jafarov, Charles J. Abolt

Department of Earth and Atmospheric Sciences: Faculty Publications

As ice wedge degradation and the inundation of polygonal troughs become increasingly common processes across the Arctic, lateral export of water from polygonal soils may represent an important mechanism for the mobilization of dissolved organic carbon and other solutes. However, drainage from ice wedge polygons is poorly understood. We constructed a model which uses cross-sectional flow nets to define flow paths of meltwater through the active layer of an inundated low-centered polygon towards the trough. The model includes the eects of evaporation and simulates the depletion of ponded water in the polygon center during the thaw season. In most simulations, …

Diagnostic Analysis Of Bank Storage Effects On Sloping Floodplains, Xiuyu Liang, Vitaly Zlotnik, You‐Kuan Zhang, Pei Xin

Department of Earth and Atmospheric Sciences: Faculty Publications

Stream bank storage effects during floods have received limited attention, despite the significant role of such floods in aquifer water budgets. One reason is the complexity of geometry of the problem, which commonly has been treated numerically. Using a simple model in a domain with moving boundary, a semianalytical solution for bank storage effects is proposed to account for stream stage hydrograph, floodplain slope, and aquifer parameters. The results extend classic solutions by Cooper and Rorabaugh (1963, https://doi.org/10.3133/wsp1536J) for idealized vertical streambanks but applied to realistic floodplain cross sections. The accuracy of the semianalytical solution is verified by a one‐dimensional …

The Prairie Post Quarterly Newsletter Of The High Plains Regional Climate Center- January 2020, Natalie Umphlett, Rezaul Mamood, Paul Flanagan, Logan Winters, Kierstin Blomberg, Emilee Lachenmeier, Crystal J. Stiles

Prairie Post: Quarterly Newsletter of the High Plains Regional Climate Center

Inside this issue:

Message from the director........................................1

Staff spotlight...........................1

Year in review............................2

Updates on engagement and tools.....................................3

Observer awards......................3

Overview of regional climate conditions..................................4

AMS annual meeting.............5

Recent and upcoming travel and activities.............................6

Altitude Of The Potentiometric Surface In The Mississippi River Valley Alluvial Aquifer, Spring 2018, Virginia L. Mcguire, Ronald C. Seanor, William H. Asquith, Anna M. Nottmeier, David C. Smith, Roland W. Tollett, Wade H. Kress, Kellan R. Strauch

United States Geological Survey: Publications

The Mississippi River Valley alluvial (MRVA) aquifer is an important surficial aquifer in the Mississippi Alluvial Plain (MAP) area. The aquifer is generally considered to be an unconfined aquifer (fig. 1; Clark and others, 2011), and withdrawals are primarily used for irrigation (Maupin and Barber, 2005). These groundwater withdrawals have resulted in substantial areas of water-level decline in parts of the aquifer. Concerns about water-level declines and the sustainability of the MRVA aquifer have prompted the U.S. Geological Survey (USGS), as part of the USGS Water Availability and Use Science Program and with assistance from other Federal, State, and local …

Water Quality Of Groundwater Used For Public Supply In Principal Aquifers Of The Western United States, Celia C. Rosencrans, Marylynn Musgrove

United States Geological Survey: Publications

Groundwater provides nearly half of the Nation’s drinking water. As the Nation’s population grows, the importance of (and need for) high-quality drinking-water supplies increases. As part of a national-scale effort to assess groundwater quality in principal aquifers (PAs) that supply most of the groundwater used for public supply, the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Project staff sampled six principal aquifers in the western United States between 2013 and 2017: (1) the Basin and Range carbonate-rock aquifers, (2) Basin and Range basin-fill aquifers, (3) Rio Grande aquifer system, (4) High Plains aquifer, (5) Colorado Plateaus aquifers, and (6) Columbia …

Estimated Groundwater Withdrawals From Principal Aquifers In The United States, 2015, John K. Lovelace, Martha G. Nielsen, Amy L. Read, Chid J. Murphy, Molly A. Maupin

United States Geological Survey: Publications

In 2015, about 84,600 million gallons per day (Mgal/d) of groundwater were withdrawn in the United States for various uses including public supply, self-supplied domestic, industrial, mining, thermoelectric power, aquaculture, livestock, and irrigation. Of this total, about 94 percent (79,200 Mgal/d) was withdrawn from principal aquifers, which are defined as regionally extensive aquifers or aquifer systems that have the potential to be used as sources of water of suitable quality and quantity to meet various needs. The remaining 6 percent (5,400 Mgal/d) was withdrawn from other, nonprincipal aquifers in the United States. Sixty-six principal aquifers belonging to 5 major lithologic …

Estimating Abiotic Thresholds For Sagebrush Condition Class In The Western United States, Stephen Boyte, Bruce K. Wylie, Yingxin Gu, Donald J. Major

United States Geological Survey: Staff Publications

Sagebrush ecosystems of the western United States can transition from extended periods of relatively stable conditions to rapid ecological change if acute disturbances occur. Areas dominated by native sagebrush can transition from species-rich native systems to altered states where non-native annual grasses dominate, if resistance to annual grasses is low. The non-native annual grasses provide relatively little value to wildlife, livestock, and humans and function as fuel that increases fire frequency. The more land area covered by annual grasses, the higher the potential for fire, thus reducing the potential for native vegetation to reestablish, even when applying restoration treatments. Mapping …

Applied Injected Air Into Subsurface Drip Irrigation: Plant Uptake Of Pharmaceuticals And Soil Microbial Communities, Matteo D'Alessio, Lisa M. Durso, Clinton Williams, Christopher A. Olson, Chittaranjan Ray, Ellen Paparozzi

Nebraska Water Center: Faculty Publications

The growing global food security crisis is complicated by the need for increased crop production with less arable land and limited water resources. Reuse of treated wastewater for agricultural irrigation is becoming more common, often paired with other conservation measures such as subsurface drip irrigation (SDI). Passively injecting air into SDI systems increases crop yields and overcomes root zone wetting issues. However, when used with treated irrigation water, contaminants in the water might be taken up by the crops. This paper investigates the impact of air-injected water containing caffeine, carbamazepine, and gemfibrozil on plant uptake and soil microbial communities in …

Droughtscape, Winter 2019, Cory Matteson

DroughtScape: Quarterly Newsletter of the National Drought Mitigation Center (2007–2023)

Contents

From the Director

4th quarter climate summary

2019 year in review summary

4th quarter drought impact summary

2019 drought impact summary

New form helps ‘see more’ drought

Landscape photographers invited to submit photos

Helping Central and South America planning

Database includes more drought planning

Assessing The Hierarchy Of Long-Term Environmental Controls On Diatom Communities Of Yellowstone National Park Using Lacustrine Sediment Records, Victoria Chraibi, Sherilyn C. Fritz

Department of Earth and Atmospheric Sciences: Faculty Publications

An ecosystem’s ability to maintain structure and function following disturbance, defined as resilience, is influenced by a hierarchy of environmental controls, including climate, surface cover, and ecological relationships that shape biological community composition and productivity. This study examined lacustrine sediment records of naturally fishless lakes in Yellowstone National Park to reconstruct the response of aquatic communities to climate and trophic cascades from fish stocking. Sediment records of diatom algae did not exhibit a distinct response to fish stocking in terms of assemblage or algal productivity. Instead, 3 of 4 lakes underwent a shift to dominance by benthic diatom species from …

Surficial Fractures In The Navajo Sandstone, South-Western Usa: The Roles Of Thermal Cycles, Rainstorms, Granular Disintegration, And Iterative Cracking, David Loope, Garrison R. Loope, Caroline M. Burberry, Clinton Rowe, Gerald C. Bryant

Department of Earth and Atmospheric Sciences: Faculty Publications

Deep (> 5 m) sheeting fractures in the Navajo sandstone are evident at numerous sites in southern Utah and derive from tectonic stresses. Strong diurnal thermal cycles are, however, the likely triggers for shallow (< 0.3 m) sheeting fractures. Data from subsurface thermal sensors reveal that large temperature differences between sensors at 2 and 15 cm depth on clear summer afternoons are as great as those that trigger sheeting fractures in exposed California granite. Extensive polygonal patterns in the Navajo sandstone are composed of surface-perpendicular fractures and were produced by contractile stresses. Numerous studies have shown that porewater diminishes the tensile strength of sandstone. Based on our thermal records, we propose that cooling during monsoonal rainstorms triggers polygonal fracturing of temporarily weakened rock. On steep outcrops, polygonal patterns are rectilinear and orthogonal, with T-vertices. Lower-angle slopes host hexagonal patterns (defined by the dominance of Y-vertices). Intermediate patterns with rectangles and hexagons of similar scale are common. We posit that outcropping fractures are advancing downward by iterative steps, and that hexagons on sandstone surfaces (like prismatic columns of basalt) have evolved from ancestral orthogonal polygons of similar scale. In lava flows, fractures elongate intermittently as they follow a steep thermal gradient (the source of stress) as it rapidly moves through the rock mass. In our model, a steep, surficial thermal gradient descends through unfractured sandstone, but at the slow pace of granular disintegration. Through time, as the friable rock on stable slopes erodes, iterative cracking advances into new space. Hexagonal patterns form as new fractures, imperfectly guided by the older ones, propagate in new directions, and vertices drift into a configuration that minimizes the ratio of fracture length to polygon area.