Earth Watching,
Landing
and Flowing 地球观察・落地・流动 Observer la Terre, atterrir, et suivre le courant

Mongolia is three hydrological worlds at once: an Arctic drainage (Selenge → Baikal → Yenisei), a Pacific drainage (Onon and Kherlen → Amur), and a vast closed interior — the Great Lakes Depression and the Gobi, where rivers end in salt lakes or in the ground. Flow is snow- and monsoon-pulsed: most annual runoff arrives in a few summer weeks; rivers freeze to the bed in reaches through winter.

三个水文世界并存：北冰洋水系（色楞格→贝加尔湖）、太平洋水系（鄂嫩、克鲁伦→黑龙江）、以及广阔的内流区（大湖盆地与戈壁）。径流高度集中在夏季数周，冬季部分河段冻至河床。

The relief is high but gentle: headwaters at 2,500–3,500 m fall to a 456 m lake over more than a thousand kilometres of steppe. That means low gradient, low erosive power, and modest hydropower head compared with mountain nations — but also long, slow, braided reaches, wide floodplains and heavy sediment residence. Ulaanbaatar sits on the Tuul at ~1,300 m, on gravels that are the city's aquifer.

高海拔但低坡降：源头 2,500–3,500 米，千余公里草原后落至 456 米的贝加尔湖。落差势能有限、水力发电潜力中等，但河道漫长、辫状、洪泛区宽阔。乌兰巴托位于图勒河谷 1,300 米，城市取水依赖河谷砂砾含水层。

Risk is concentrated, not distributed: summer flash floods in ger districts on Ulaanbaatar's unpaved slopes, where runoff coefficients are high and drainage informal, plus ice-jam and snowmelt floods on the Selenge and Kherlen. The compound hazard that actually kills livestock and livelihoods is the drought → dzud couplet, as in the 2023–24 winter.

Two signatures dominate the public record: untreated and under-treated municipal wastewater entering the Tuul below Ulaanbaatar, and placer gold mining that has physically dismantled riverbeds and riparian zones in the Tuul, Orkhon and Zaamar reaches — turbidity, channel destruction, mercury and abandoned pits. Mining and coal water demand competes directly with the Gobi's fossil groundwater.

Mongolia's riparian corridors are among the least fragmented in Asia — the Onon–Balj and Mongol Daguur complexes, taimen rivers in Khövsgöl, and Ramsar wetlands on the Selenge delta. The country protects roughly a fifth of its territory and has publicly committed toward the 30% pathway. The Selenge remains a free-flowing river of global significance; that is the single biggest asset in this landing.

Not irrigation-dominated: livestock (a herd that passed 60 million head) grazes the water's edge, and the transformation pressure comes from mining, coal-to-chemicals and urban water supply. Industry transformation here means dry cooling, tailings water recycling, and rehabilitating rivers that mining licences already crossed.

Spain is the most heavily engineered water system in Western Europe: over a thousand large dams, inter-basin transfers (Tagus–Segura), and a national reservoir network reported daily. Rainfall is bimodal in space — an Atlantic north with surplus, a Mediterranean south-east in structural deficit where demand no longer waits for rain but for desalination and reuse.

西欧水利工程化程度最高的国家之一：上千座大坝、跨流域调水（塔霍—塞古拉）。北部大西洋侧盈余，东南地中海侧结构性缺水，需求已从"等雨"转向海水淡化与再生水。

A high central Meseta (600–800 m) rimmed by ranges, drained by long rivers that incise sharply at the edges — hence Spain's exceptional hydropower and pumped-storage head, and hence the danger. Short Mediterranean ramblas and barrancos fall from 1,000 m to the sea in tens of kilometres: dry for years, then delivering a wall of water in hours.

中央高原（600–800 米）四周被山系包围，河流在边缘急剧下切——既造就了优异的水电与抽水蓄能落差势能，也带来风险：地中海侧短小的干河谷可在几十公里内落差千米，数年干涸，数小时成灾。

The 29 October 2024 DANA over Valencia is now the reference event: more than 700 mm in 24 hours at Turís, the Poyo ravine and Magro overwhelming urbanised floodplain, and over 220 deaths. It exposed the real gap — not the absence of maps (Spain's flood-risk mapping is mature) but warning-to-action time in places built where water used to go.

Nitrate is the defining pollutant: intensive irrigation in Campo de Cartagena drove the Mar Menor lagoon into repeated anoxic collapse and mass fish die-off. Aquifer over-abstraction — with an estimated order of a million unauthorised wells nationally — links pollution and scarcity into one problem. Doñana's marshes have contracted as the aquifer beneath them fell.

Here is Europe's brightest river story: Spain removes more river barriers than any other European country, in the hundreds since 2018, reopening spawning routes on the Mijares, Duero and Cantabrian rivers. And in 2022 the Mar Menor became the first ecosystem in Europe granted legal personhood by popular legislative initiative. Against that, the Ebro delta — 320 km², much of it under half a metre above sea level — is sediment-starved by upstream dams and losing ground to the sea.

Europe's orchard and greenhouse, running on the driest water. The transformation underway is real but uneven: pressurised and deficit irrigation, treated-wastewater reuse under the EU reuse regulation, solar-powered desalination on the south-east coast, and pumped hydro repurposing old head. The unresolved question is whether efficiency gains are returned to the river or reinvested in more hectares.

Abundance with terrible timing. Some of the highest runoff per square kilometre in the world, delivered by orographic westerlies onto a mountain spine, then drained east through dry, irrigated plains. The result is a country that floods and irrigates in the same season, and where lakes (Taupō, Wānaka, Hāwea, Wakatipu) act as the national reservoir, storing both water and electricity.

水量丰沛但时机严酷。单位面积径流量全球领先，西风带地形抬升降雨，向东排入干燥的灌溉平原：同一季节里既防洪又灌溉；陶波、瓦纳卡等湖泊实际上是国家级的水库与"电池"。

The steepest hydrology in this landing. Catchment response is measured in hours, not days; sediment yields are among the highest on Earth in the erodible East Coast hill country. That same relief makes hydro the backbone of an electricity system that is roughly 80–90% renewable — the 落差势能 is literally the national grid, and a dry year in the Clutha headwaters is a national energy event.

四地中坡降最陡：汇流响应以小时计；东岸易蚀丘陵输沙量居世界前列。同样的落差支撑起以水电为骨干、约 80–90% 可再生的电力系统——落差势能就是国家电网，克鲁萨源头的枯水年即是全国性的能源事件。

Cyclone Gabrielle (February 2023) is the calibration point: eleven deaths, whole valleys in Hawke's Bay buried in silt, and damage estimates in the many billions of NZ dollars — the country's costliest weather event. Three weeks earlier Auckland took its wettest day on record. Ex-tropical cyclones reaching further south is the risk signal, and it lands on rivers that already move mountains of sediment.

Not industrial — diffuse. Nitrogen, phosphorus, sediment and E. coli from pastoral intensification put a large share of monitored lowland river length in the poorest national bands for swimmability and nitrate toxicity, while groundwater nitrate rises in Canterbury and Southland. Reporting is unusually honest and public; that transparency is itself an asset.

The world's most important legal innovation in river governance happened here. The Te Awa Tupua (Whanganui River Claims Settlement) Act 2017 recognised the Whanganui as a legal person with its own standing and guardians. On the Waikato, a Māori–Crown co-governance settlement (2010) established a binding Vision & Strategy and a long-term clean-up fund. On the ground: tens of thousands of kilometres of stock-excluded, replanted riparian margin — the largest voluntary riverbank restoration programme in the southern hemisphere.

Dairy, horticulture and forestry are the economy and the load. Transformation levers actually in motion: nitrogen caps and fertiliser limits, effluent-system upgrades, wetland reconstruction as nutrient interception, and precision-fertiliser using satellite and drone data. The countervailing force is policy reversal risk — freshwater rules that were tightened are being loosened again.

Mexico's water and Mexico's people are on opposite sides of the map. The Grijalva–Usumacinta in the wet south-east carries the largest discharge in the country through the least-populated basin; the arid north and the central plateau host the cities, the industry and the irrigation districts. Over 100 aquifers are formally over-exploited.

水与人分布相反：东南部的格里哈尔瓦—乌苏马辛塔径流量最大、人口最少；干旱的北部与中部高原集中了城市、工业与灌区。全国已有一百多个含水层被正式认定为超采。

Two Sierra Madre escarpments frame a high plateau, giving steep, high-head rivers — the Grijalva alone carries a cascade of four large dams supplying a major share of national hydropower. But the central plateau's defining fact is negative head: Mexico City sits at 2,240 m in a basin with no outlet, so water must be pumped over a kilometre uphill from the Cutzamala and sewage pumped out through tunnels. Both directions cost energy no other megacity pays at this scale.

东西马德雷山系夹着中央高原，河流坡陡、水头高（格里哈尔瓦四级大坝贡献可观水电）。但高原的关键事实是"反向落差"：墨西哥城位于 2,240 米的无出口盆地，供水需从库察马拉抬升逾千米，污水需靠隧道外排——两个方向都在支付其他特大城市不必支付的能耗。

Flood and drought arrive in the same year. Tabasco's Grijalva lowlands flood almost structurally; Hurricane Otis (Acapulco, 2023) and Tula's 2021 flood showed how fast urban river reaches fail. Meanwhile the 2024 Cutzamala low point took reservoirs toward a quarter of capacity and put "day zero" into national conversation. Subsidence in Mexico City is now itself a flood driver, cracking drains and reversing gradients.

The Río Santiago at El Salto–Juanacatlán is one of the Americas' most documented industrial-and-municipal pollution cases, with human-rights findings attached to it; the Atoyac (Puebla–Tlaxcala) and the Tula (receiving Mexico City's wastewater for irrigation) complete a trio that is now named in national sanitation programmes. Untreated municipal discharge remains the largest single load.

Two genuinely strong assets. First, the 2018 water-reserve decrees: environmental flows legally reserved across 295 basins, covering a majority share of national runoff — one of the largest environmental-flow allocations ever enacted anywhere. Second, the south-east's intact riparian and wetland complexes: Pantanos de Centla (over 300,000 ha, Ramsar) and the Usumacinta–Lacandona corridor, still among the great freshwater ecosystems of the Americas.

Irrigation districts in the north-west and Bajío, and export manufacturing along the border, both drawing on stressed basins. Transformation in motion: the national water plan's sanitation of the Santiago, Atoyac and Tula; technified irrigation and volumetric metering; nearshoring pushed to prove water availability before it builds. Unresolved: the 1944 Rio Grande treaty deliveries, where cycle deficits have become a recurring bilateral flashpoint.