Driven by the EU agricultural market, groundwater-irrigated agriculture in Spanish water-scarce regions has switched from alfalfa and maize to less water-intensive, high-return crops such as grape and olive. On the one hand, the Spanish government and agricultural propaganda departments make full use of the abundant seawater resources for desalination [2] . On the other hand, Spain promotes the water-saving technology of facility agriculture, by the adjustment of greenhouse structure, soil improvement, water-saving irrigation, etc. In recent years, the benefits of its greenhouse horticultural crops per ton of water are as high as 12.5 Euros, equivalent to 92.5 Yuan [3] . At the same time, irrigation infrastructure of Spain, including water storage tanks, water purification stations, water pipelines, irrigation pump stations and other equipment investment, in addition to the facilities required by farmers in the fields, is basically carried out by the European Union and the central and local governments of Spain undertake [4] , and most of them are free of charge.

Cai Hongyi (2017) and other scholars compared the construction of agricultural water-saving irrigation in Spain, Japan and the United States and argue that in order to solve the contradiction between the supply and demand of agricultural water resources, the Spanish government has launched a series of reconstruction plans in irrigation to improve agricultural production efficiency and develop modern irrigation. In these plans, the government and civil organizations will jointly invest in the old irrigation areas reforming, replacing traditional canals with pipeline, transforming surface irrigation technology into mechanized irrigation technology, and giving priority to the transformation of aging key projects and the upgrading of modern technology in field projects. Gradually reduce the number of new large-scale irrigation projects, and transform the original traditional extensive expansionary development path into a technology upgrade based on engineering technology content [5] . Most of the farmland in Spain has adopted advanced irrigation technologies such as sprinkler irrigation, drip irrigation and infiltration irrigation, and has achieved automation, digitization, and remote control in soil moisture monitoring, precipitation monitoring, field experiments, and crop planting [6] .

Almeria (European “vegetable basket”), located in the south of Spain. Although it is coastal, it is on the leeward slope of the westerly wind and is affected by the subtropical high pressure in the season. It is dry year round, and it is only 213 mm of precipitation, so, water bills are very expensive. In order to cut costs, the Spanish government has adopted a series of water-saving measures in agricultural production: They require farmers to build rain pools、build rain storage pipes on the roof of greenhouses and support farmers to use drip irrigation and fertilization systems; implements national policies; relies on geographic advantages, desalinates Mediterranean seawater; drills wells or canals in mountainous areas... At the same time, in order to slow down water evaporation and follow the concept of development of water-saving agriculture, all farmland in Almeria area is plastic and steel frame greenhouses, to ensure the humidity required by crop growth to a certain extent and realize water saving in the process of crop water conversion.

When inspecting the facility horticulture industry in Almeria, Spain, Bao Shunshu (2019) pointed out: the soil parent material in Almeria is sand and gravel, local farmers use sand, organic fertilizer and clay to make the new soil profile show a “sandwich” structure: the surface layer is the imported sand, the middle layer is fermented organic fertilizer, and the lower layer is clay [7] . The upper layer of sandy soil has good water permeability and air permeability, which can quickly absorb water and fertilizer to prevent ground runoff; the middle layer of organic fertilizer can enhance the fertilizer, water retention and buffer capacity of soil, reduce the fixed loss of nutrients, thereby creating good soil conditions for the growth of crops; the lower layer of clay soil can maintain water and fertilizer to avoid nutrient, water seepage. The “sandwich” structure (as shown in Figure 1) scientifically promotes the water cycle of crop roots under limited natural and geographical conditions from a new perspective, maximizes the efficiency of irrigation water use, and improves the unilateral aspect of water output in agricultural production to achieve the development goal of water-saving agriculture.

Angel Paniagua Mazorra (2001) pointed out that agricultural water-saving irrigation management system of Spain adopts appropriate decentralization between the central and local governments. The government is mainly responsible for formulating agricultural water-saving irrigation policies, implementing specific water-saving irrigation construction projects, supervising and guiding large-scale water source projects, construction and management of key water delivery projects; the main responsibility of the civil organization—water user association is to be responsible for the construction and management of irrigation projects [8] . FG Gomez (2012) and Zhu Qingyun (2012) discussed the “European Water Framework” in detail, arguing that the Spanish agriculture generally adopts the mode of forming farmers’ associations for farmers’ self-management, and farmers’ associations and government departments obtain water use through negotiation. The right to use water is distributed democratically within the farmers’ association to popularize the idea of water conservation [9] .

As the fourth-ranked developed country in the world, Germany’s GDP is three times that of China’s total, but it “feeds the whole of Germany with 2% of the agricultural population”. A strong industrial system drives strong agricultural mechanization, and Germany’s digital agriculture is comprehensive, so its water-saving agriculture is also fully intelligent [13] . For example, Wang Shuting (2020) pointed out that the temperature, soil quality, precipitation, humidity and other data of each acre of arable land should be transmitted to the cloud for processing on the cloud platform, and then the generated relevant soil data will be processed on the cloud platform. The data of moisture content and required moisture are sent to intelligent large-scale agricultural machinery according to the weather conditions in a short time (with or without rainy weather), and they are instructed to carry out refined operations, including irrigation or moisturizing, heat preservation, etc [14] .

Rain-fed agriculture attaches great importance to the utilization rate of precipitation, and the relevant agricultural departments of Germany have adopted various measures to improve the soil’s ability of absorbing precipitation. For farmers, they are more concerned about the available field water capacity, that is, the amount of water that can be stored in the field soil and absorbed and utilized by crops in the precipitation. In Chen Zhangquan (2017)’s research on the protection and sustainable utilization of agricultural water and soil resources in Germany, he pointed out that the German government agricultural department will release information on the water content of the crop plough layer soil, but many farmers will install soil moisture monitoring instruments on their farms to monitor real-time measurement of soil moisture changes [15] . At the same time, 100% of Germany’s agriculture has achieved water-saving irrigation [16] , of which, sprinkler irrigation accounts for about 80%, mainly used for field crops such as wheat and potatoes; drip irrigation accounts for about 20%, mainly used for vegetables and fruits. Due to the complete elimination of surface irrigation irrigated agriculture has high water use efficiency.

It is worth mentioning that another important reason for the continuous improvement of agricultural water use and utilization in Germany is that the government mainly promotes farmers to adopt advanced irrigation technology through market means—water price regulation to achieve the purpose of water saving. In 2017, Dortmund’s price of water is as high as 2.9 Euros (21.5 Yuan) per cubic meter of water. Even water companies and agricultural irrigation and water pipeline companies do everything possible to prevent leakage of water pipelines. In addition to, the price of crops in Germany is not high, the price of water-saving irrigation equipment is high. Farmers can only reduce the cost of irrigation water to reduce planting costs. This has formed a forced water-saving irrigation mode, which makes the agricultural water-saving concept deeply rooted in the hearts of the people.

In order to protect groundwater, Germany has not only formulated strict production license application procedures for agricultural material production enterprises, but also made clear and strict regulations on fertilization and spraying pesticide of farmers. When farmers use certain pesticides or fertilizers on crops, they must keep records so that the relevant departments can check and control them. B. Köstner (2014) emphasized many times in the study of plant production: Germany promulgated fertilization regulations as early as 1996, stipulating that any fertilization behavior is prohibited from November 15^th to January 15^th of the following year. Phosphate fertilizer, Potassium fertilizer once every three years to avoid crops not absorbing and affecting groundwater and soil [17] . Zeng Zhe (2020) stated: According to the policy, if farmers contribute to the protection of the natural environment and biodiversity, engage in agricultural production activities in a sustainable manner, they can receive green compensation payments. Once violations of cross-compliance (highlighting the protection of water, soil, and biodiversity) are found in agricultural production, green direct payment funds will be deducted ranging from 1% to 20% as appropriate. Scientifically coordinate the amount of water intake and replenishment. When extracting groundwater as a source of irrigation water, the extraction rate should be determined according to the rate of groundwater replenishment to ensure that the groundwater level does not drop and achieves a balance between in and out.

Italy regards water-saving irrigation as the core of national water-saving agricultural development. It is conceivable that Italian water-saving irrigation is quite successful. Sun Bo (2002) and Wu Zhengang (2002) pointed out in their research that sprinkling and micro-irrigation technologies had been introduced from the United States and Israel in the 1960s, and it only took about 10 years to overcome the shortcomings of truss-type sprinkling irrigation and micro-technology. And the reel type sprinkler irrigation machine was also developed, which solved the water-saving irrigation problem of field crops (wheat, corn, soybean, etc.) [19] . Up to now, the reel-type sprinkler irrigation equipment in Italy has been quite mature, and the reel-type sprinkler irrigation in the domestic plain area has reached 60% - 70%.

With the continuous progress of science and technology and industrialization, Italian farms are constantly adding water-saving irrigation facilities (see Table 1). At present, the drainage and irrigation channels in Italy are 181,312 kilometers long. The total irrigation area under the management of the National Irrigation Committee has reached 3.3 million hectares. Among them, the sprinkler irrigation area is 1.683 million hectares, the micro-irrigation area is 660,000 hectares, and the total water-saving irrigation area is 2.343 million hectares, accounting for about 71% of the actual irrigated area (see Table 2). And the coil-type sprinkler that is used for sprinkler irrigation saves water and energy significantly. It only needs 450 cubic meters of water per hectare at a time. In Europe, crops

Source: The author’s self-made according to the “Italy ‘Agricultural Water-Saving Irrigation Technology Training’ Report”.

Source: The author made his own according to the “Italy ‘Agricultural Water-Saving Irrigation Technology Training’ Report”.

such as wheat and corn are generally sprinkled 5 times per season, and the annual water consumption is 2250 - 3000 cubic meters per hectare, less loss, high utilization rate of irrigation water, it can save more than 50% of water.

In addition, Italy made full use of EU funds and cooperated with the implementation of the EU’s seven-year plan (2014-2020) in 2013. Zhang Manjie (2014) emphasized in her research on the development of Italian agriculture: Italy has always used EU funds to provide farmers with a lot subsidies from every aspect [20] . In terms of precision agriculture, the key is precision irrigation and fertilization technology [21] , which has been used in 5% of Italian farms. The research of Zhou Xiaoping (2008) shows that: Italy uses the advanced PLC automatic control technology to combine with irrigation (as shown in Figure 2) to achieve the purpose of intelligent, accurate and even water supply [22] . At the same time, sensors for monitoring water, humidity, and soil composition are arranged in the soil, the remote monitoring system is used to analyze, calculate the amount of water to be irrigated, analyze the amount of fertilizer required by the soil, and give the prescription map of each piece of land, so as to ensure accurate irrigation, use fertilizers and pesticides. This way can achieve the purpose of saving water and ensure the water quality of irrigation [23] .

However, no matter whether the irrigation method at the end is drip irrigation or sprinkler irrigation, the water quality requirements of this precision irrigation technology cannot be ignored. Because in addition to the high investment cost, the biggest disadvantage of sprinkler and drip irrigation is that it is easy to block. However, this irrigation technology has strong terrain adaptability, so experts are working hard to make up for its shortcomings, such as adding lamination filters or choosing sprinklers with strong anti-clogging and stable performance.

The Kingdom of Denmark is located in northern Europe and consists of more than 480 islands. The average annual precipitation in the territory is 600 mm. The climate is changeable, and drought often occurs in spring and summer. Such climatic conditions are not conducive to agricultural production [24] , but Danish agriculture still has strong competitiveness in the international market, its agricultural area accounts for two-thirds of the territory of country. Agriculture is based on animal husbandry, water accounts for more than 70% of total water consumption. Its livestock and poultry breeding industry is mainly pig raising. Each Dane can share nearly 6 pigs on average. It is the only country in the world with more pigs than people [25] , and each pig consumes about 200 L of water.

Due to the scarcity of surface water, Denmark uses groundwater almost completely. Over-exploitation of groundwater will lead to many irreversible environmental problems: the groundwater level continues to decline, the groundwater level funnel continues to expand, and the groundwater volume decreases, resulting in land subsidence, ground collapse, seawater intrusion and other serious consequences of ecological environment deterioration. The United States, China, Japan and other countries and regions have already learned a bloody lesson. Therefore, the relevant departments of the Danish government pay special attention to the limit and the efficiency of groundwater use.

Under the strict water management, Danish water leakage rate is far below the international average. Some countries have a water leakage rate of 40%, while Danish water leakage rate is only 8%. In addition to, Denmark will regard the recycling of groundwater and wastewater treatment as the heart of water-saving agriculture. For example, in the research of Cui Ningbo (2020) and Ba Xuezhen (2020), it was mentioned that in order to protect groundwater, Denmark has strictly controlled the use of pesticides since the 1980s, and strived to minimize all kinds of pollution [26] . Slaughterhouses and farms in Denmark, as well as cooperative enterprises in the production and processing of agricultural products, purify and recycle wastewater from production lines in order to prevent nitrogen loss and contamination of groundwater resources [27] , the fertilizer for pigs should be fermented and applied to the field at the specified time (twice in spring and autumn), while the organic matter in the wastewater is concentrated in the sludge, the phosphorus extracted from the wastewater or sludge can become fertilizers. The remaining mineral and organic components in the sludge can be used as soil conditioners or additional fertilizers [28] . Guo Hongpeng (2015) and Xu Beichun (2015) did in-depth research on the above. In this way, while protecting and saving groundwater resources, a circular organic water-saving agricultural system is also formed.

At the same time, Denmark has the highest water price in the world, with an average of 8 euros per cubic meter. Industrial, agricultural and living water use one price, 34% of the high water price is used for water supply and 66% is used for wastewater treatment [29] . Guo Xin’s (2020) study shows that the cost of wastewater collection and treatment is usually twice the cost of water abstraction and water supply, adhering to the principle of “who pollutes, who treatments” [30] . For decades, the price of water in Denmark has continued to rise. Even so, the productivity and efficiency of Danish agriculture have not been affected. Especially as a major agricultural country, its agricultural efficiency is still leading the world, and its experience in water-saving agriculture is of forward-looking significance.